Just A Few Cup Sizes

by 555randomness

Rating: mature
Tags: Twilight Sparkle, Anthro, My Little Pony: Friendship is Magic, Rainbow Dash, Sex, Random
Length: 3666

Twilight tries to help Rainbow Dash fulfill her most hidden fantasies, both get more than they bargained for.

Rainbow Dash accidentally reveals her guiltiest fantasy during some drunken ravings. Twilight Sparkle tries to help her fulfill this fantasy using a new spell. However, both of them end up getting far more than they bargained for.

Warning, this story contains: Breast Expansion, Butt/Hip/Thigh Expansion, FutaXFemale, minor Cock Growth, and lastly some light Cum Inflation.

Cover art belongs to fenrox.

Finally, special thanks to Megapone for proofreading this.

Published 2015-10-24 17:50:02 UTC
Last updated 2015-10-24 17:50:02 UTC

Just A Few Cup Sizes

Rainbow Dash went through the streets of Ponyville at a light jog, enjoying the cool breeze in her hair. She was heading over to Twilight's to help her "test out something new.”She found herself a bit nervous; usually whenever Twilight was trying out something new it meant trouble. To be fair though, Twilight's experiments never caused any real harm, and she always fixed anything that went haywire.

Dash arrived at the library and knocked on the door. She could only wonder what Twilight had in store as she waited for an answer. Twilight soon answered the door, wearing an unassuming button-up shirt, skirt, and lab coat over the top of it all. "Hello, Rainbow Dash. Thanks for coming. You can go ahead and step inside—I just have to finish up something real quick."

Rainbow Dash casually followed Twilight inside. "You seemed pretty excited the last time we talked. What's all this about?"

Twilight disappeared for a moment before returning into the main room, minus her lab coat, to answer Rainbow's question. "Alright, this might seem awkward to ask, but it's relevant. Do you remember that one time when you told me that you've always wished for a curvier figure?"

Rainbow Dash blushed slightly and began to stumble over herself. "Uh...y-yeah. Well… You see, I was really drunk at the time. I was hoping you didn't remember me saying that.."

Twilight smiled reassuringly. "It's nothing to be ashamed of. I understand, and in fact I've been working on a spell that might help with that. If it works, it can be used for other mares who have similar desires. It'll also let me learn a counterspell to help those with the opposite problem. So, what do you say?"

Rainbow mulled over the idea. As much as she hated to admit it, the idea of having larger breasts and a bigger butt had always been a guilty fantasy of hers. She had always found herself feeling a bit jealous whenever she saw a more endowed mare walking down the street. Due to her athletic lifestyle and getting short-changed by puberty she had believed this fantasy would be forever out of reach, but here before her was a chance to make it actually happen. Part of her was screaming that this was a bad idea, but in the end she decided that Twilight's offer was too rare and tempting to ignore. "Alright, we'll try it. Keep the changes small though. I don't want this going overboard."

Twilight nodded, beaming at Dash's response. "Don't worry. you should only gain a few cup sizes at most, and maybe a pants size or two." With that Twilight began charging up her spell, with Rainbow Dash bracing herself for whatever came next. Soon her body lit up with magic and glowed briefly before the effect went away.

Rainbow quickly looked over her body for any signs of change, but didn't notice anything. "I don't feel any different yet, when's this thing supposed to kick in?" About as soon as she spoke she began to notice a sensation of warmth near her chest. That's when it hit her.

She let out a light moan as the sensation intensified. Two bulges began to push out against the top of her shirt, growing rapidly. She cupped her hands to her chest as her breasts began to swell, already feeling overwhelmed by the growth. It wasn't just confined to her upper body; before long she began to feel similar magic working lower down her body. Her running shorts were starting to become slightly tight as her previously flat flank began to push out and take up a more rounded shape. Meanwhile her narrow hips began to stretch and widen themselves, creating a slightly feminine curve at her waistline. Even her thighs began to feel a bit thicker.

All the while Twilight was watching these developments with keen interest. Dash's shirt was starting to ride up and reveal part of her midriff; her enlarging breasts now demanding more fabric to cover them. Similarly, Rainbow's shorts were slowly clinging tighter and tighter to her body, covering an increasingly smaller amount of her flesh as she grew. Twilight bit her lip and tried to keep a reserved look, but it was hard to deny how erotic of a sight what she was witnessing.

Rainbow Dash bent over slightly as the growth seemed to intensify and her bust continued becoming heavier. It seemed that the pleasure she was receiving from the growth was getting stronger as well. Her wings flared out behind her and she continued letting out gentle moans with each pulse of growth. Her shirt had now ridden up so far it barely covered any of her belly, and a noticeable amount of cleavage was beginning to show. It was hard to think straight, but if Rainbow had to guess she'd put herself easily in the D-cup range, if not double D. Her shorts were becoming inadequate as well, her rounded bubble butt and widening hips causing rips and tears to form. Meanwhile her thickened thighs were becoming more visible, as the shorts were quickly becoming short shorts and only covered half of what they used to. Despite how intense it all was, she knew that this growth could quickly get out of hand if it wasn't stopped.

"Uh Twilight? Shouldn't the spell be slowing down by now...and not speeding up?"

"It should have," Twilight responded nervously. "I don't know why it's still going."

"Can't you just cancel it then?"

"It could be dangerous to try interfering with the spell while it's in effect; we should just let it run it's course for now. I'm sure there can't be that much juice left to it." Rainbow grumbled in response to that answer, but was drowned out by the sounds of straining fabric that wouldn't be able to withstand her expanding figure much longer. "Uh, Rainbow. I don't want to make you uncomfortable...but you might want to take off some of those clothes if you don't want them getting ripped."

Rainbow Dash wasn't sure how to feel about the idea of stripping in front of her friend, but Twilight was right. She didn't want these clothes getting ripped, and they were becoming extremely uncomfortable anyways. She removed her t-shirt first, revealing a small sports bra that was being overwhelmed by two orbs of flesh far larger than it was designed for. Her cleavage bulged obscenely against the tight fabric, pushing closer to freedom with each spurt of growth. Next were her shorts, a task made difficult due to her now heavily rounded rear and curved hips. She slowly managed to take them off, forcing them past her thick thighs which were now coming close to rubbing together in places. She was experiencing too many other feelings to be that embarrassed about being exposed down to her underwear in front of Twilight. Part of her was worried and frustrated about how out of control this was getting, but another part of her couldn't help but feel excited and almost proud of the changes. Her self-control began to slip a little as she grabbed her rounded breasts and gave them a squeeze.

"Just a few cup sizes, huh?"

Twilight chuckled awkwardly, "Okay, I might have put a little too much power into the spell. It'll stop eventually though, and I'll shrink you back whenever it's over." On the inside she felt a bit of regret thinking about that last part. Rainbow's figure was becoming more enticing by the second, and seeing her stripped down of most her clothing was only enhancing Twilight's growing lust. She couldn't avert her eyes from watching Rainbow caress her growing breasts, the two globes of flesh jiggling and even bouncing slightly from the contact. She also couldn't ignore Dash's rapidly growing rear and how her big,rounded cheeks worked to slowly swallow more of her panties, making the underwear seem more like a thong. Twilight became so engrossed that she almost missed Rainbow's next question.

"Why is this starting to feel...so gooood?" The question was ended by a lustful moan.

"I was worried that rapid growth of certain body areas might cause pain, so I made sure the spell would make the growth pleasurable instead. I might have overdone that part as well."

Rainbow Dash was started to become lost in pleasure as the spell continued working on her. She had gone from gently rubbing and squeezing her breasts to shamelessly groping herself. Her bra was painfully tight at this point and looked ready to go at any second. She watched with growing pride as the fabric was pushed to the breaking point. As her breasts surged in size again, the straps of her bra snapped completely, and it fell down to the floor. Soon Dash's fingers found their way to her nipples, which were also growing at an equal pace. Looking down, she could hardly believe the sight before her. Each breast was now bigger than her head, and obscured any downward view. Her mounds had become larger than anything she had dreamed about. She had no idea anymore what her cup size might be, but she was definitely ahead of even the bustiest of her friends.

Meanwhile, her panties suffered the same fate as her bra. The fabric tore and fell to the ground as her huge flank burst free, wobbling wildly now that it was no longer contained. Any inhibitions or reservations that Rainbow still had faded as she slipped a hand in between her thick thighs and began to rub vigorously at her marehood.

Twilight could feel her self-control fading as well. Standing there watching Rainbow's curves grow larger and larger was leaving her in constant temptation. An intense desire was rising to grab those heavy breasts, or squeeze that increasingly plush rear. Seeing Rainbow go completely naked and begin pleasuring herself pushed Twilight over the limit. She leaned down and placed a hand on one of Dash's breasts, taking time to gently squeeze and feel the titflesh between her fingers. Rainbow shuddered from the foreign contact, and moaned with each squeeze. Twilight's hands shifted to Dash's nipples, giving her increasingly sensitive breasts the attention they deserved. This continued for a short time before Twilight dropped her hands and backed up a bit. Rainbow Dash was her friend, and she was taking advantage of her. Maybe it would be better to wait until the spell is over.

However, it seemed Rainbow Dash, had a different take on the situation. "Why did you stop?" she asked, looking at Twilight with a bit of disappointment, "I liked where that was going."

That took Twilight by surprise. "Wait, you want to keep going?" Rainbow Dash nodded. Twilight smiled, a great idea coming to mind. "Well, if we're going to go all the way with this, I've got another new spell that'll make it even better."

Rainbow watched curiously as Twilight quickly stripped down. Her body was quite modest compared to Rainbow's expanding form, but it was about to gain something that would make it stand out. Twilight's horn lit up and in a short time a brand new set of male genitals had appeared on her crotch. She was now wielding a cock nearly a foot long, and beneath it was a heavy set of testicles that were at least double the size possessed by an average stallion. She began stroking it proudly while measuring Dash's reaction.

Rainbow Dash's jaw dropped as she eyed Twilight's new equipment, and soon her head was swimming with dirty fantasies of what Twilight could do to her with that. "Oh, you like this?" Twilight asked with a teasing note. "How about we put my new addition and those new curves of yours to good use?" She gestured toward Rainbow's breasts.

Dash was happy to oblige as she wrapped her globes around Twilight's length and began to massage it. Twilight let out an audible moan as her throbbing erection was squished between the two growing mountains of flesh. More and more of her member was lost inside of Rainbow's cleavage as her breasts grew ever larger from the spell; in fact, they were nearly the size of beach balls by this point.

Rainbow panted slightly as she continued servicing Twilight's length. Her breasts were getting increasingly heavy, and even with using both hands to move them up and down the job was getting more difficult to keep up. It was also getting harder to focus as the heat between her thighs screamed for attention. It didn't help matters that every time she felt Twilight's cock throb between her breasts she imagined what it would feel like inside of her. Twilight noticed this and decided to relieve her, "Alright, Dash. I think I'm ready. Just bend over and I'll handle the rest."

Rainbow bent over on all fours as commanded, showing off her fat, rounded flank. Twilight quickly walked up and began to hot-dog her shaft in between Dash's immense ass cheeks. Meanwhile she used both hands to grope and squeeze all of her friend’s butt flesh appreciatively. Her focus had been more on Rainbow's boobs this entire time, but now she could see that her ass deserved just as much attention. Combined, her hips and flank took up more space than even her bust, and they felt like large,soft pillows.

One of Twilight's hands roamed over Dash's cutie mark. The giant lightning bolt had been stretched across the vast expanse of her rear and was bigger than her hand; perhaps even bigger than her face at this point. Twilight decided to give that over-sized cutie mark a hard spank and was rewarded as Rainbow's massive rear rippled like jello with each jiggle, giving even more pleasure to Twilight's member. Dash moaned in response and Twilight decided to spank her again. "Curvy enough yet, Rainbow?"

Rainbow Dash shook her head and was given another hard smack to the flank. "Don't worry,” Twilight said, smirking. ”The spell doesn't seem to be losing much speed yet. Who knows what size you'll be when it's over?"

Rainbow shuddered slightly, only able to imagine. She had been afraid earlier, but now she was consumed with an intense desire for more. "Come on Twi—stop teasing and get on with it."

Twilight giggled a little as she lined her cock into position. "It's so much fun though, but I guess I'll be nice." With a deep breath, she plunged her length into Rainbow Dash's folds. Soon, part of her member was pushing deep inside Rainbow's dripping wet pussy, while another part of it was wrapped by the growing cushion of her butt. Both mares moaned in sync as Twilight penetrated as far as she could before pulling back to repeat the process. It wasn't long before Twilight had built up a steady rhythm, watching with growing excitement as each thrust sent Rainbow's ass jiggling and bouncing. She could also see Dash's massive breasts swinging back and forth beneath her. It was hard to believe that that the plush,curvaceous mare she was plowing was the same mare as the slim, athletic tomboy who’d walked in not long ago.

Rainbow Dash could only moan and pant as Twilight's nearly foot-long shaft rammed in and out of her. Adding to the pleasure was the fact that her own body was still growing. The weight of her chest continued to increase as her giant tits pushed outward with each passing moment, getting ever so closer to making contact with the floor. Even more overwhelming was the almost indescribable amount of movement going on with her rear. Every thrust Twilight made sent jiggles throughout the entire curvature of Rainbow’s flank, all the way down to her tree-trunk thighs.

Twilight grunted as she worked to keep up the pace she’d established. Rainbow's ass was so massive that it was actually working as a barrier between her slit and Twilight's cock, and that barrier was only getting thicker. Originally, Twilight had worried that she might have over-equipped herself, but it was becoming clear she'd need a larger tool to keep this up. Being sure to tone down the power of the spell this time so it wouldn't get out of control, she cast the same spell on her member and watched as it began to increase both her length and girth. Within half a minute her erection had nearly doubled in size, letting her reach even deeper into Dash's inner walls.

Rainbow's moaning increased in intensity as Twilight also began thrusting much harder and faster than before. Every wobble of Dash's butt and bounce of her boobs spurred Twilight even further. She had never witnessed curves of this size before; even Princess Celestia would probably be jealous. Even more amazing was the fact that the spell, despite starting to slow down slightly, was still going. Twilight's mind was filled with fantasies of how much Rainbow Dash would grow before it wore off completely.

Rainbow Dash, too, was lost in fantasy. The spell had made her so incredibly aroused that she could barely think. All she could focus on was the increasing size of her curves and the two-foot-long stallion member railing her. She shivered slightly as her nipples began to lightly graze the floor, causing them to harden further from the cold. Soon her breasts would have her pinned to the ground, but she didn't care. She felt like an animal, moaning and howling for more. "Ooh, Twi. This is so awesome! I've never felt anything like this. I want more!"

Twilight smirked and responded by slapping Rainbow's rump hard, watching it shake and wobble like an ocean. She then used her hands to once again grope and squeeze the titanic rear, amazed by how far her fingers sunk in. Each of Rainbow's flank cheeks was now large enough to be used as a chair, and all together her ass was now up past the middle of Twilight's stomach. "I bet you were hoping something like this would happen, Rainbow. Being curvy isn't enough for you—you want to have the biggest assets in Equestria."

Rainbow gasped as her butt received another intense smack. "Mm, yes. I want to keep growing until I'm the curviest mare there is to see." Although in truth she probably already was. With breasts that were the size of yoga balls and an ass that would take up over three chairs, there likely weren’t any other females who could compete.

Rainbow's growth seemed to be winding down now, but Twilight was just getting started. She began thrusting with as much speed and power as she could muster, sending lewd,wet slapping sounds throughout the room. The repeated impacts sent Rainbow's over sized flank and thighs wobbling non-stop. Dash's breasts would probably do the same, except for the fact that they were now pressed against the ground. The two out-of-control boobs now had their owner planted to the floor, and continued to squish out beneath her like giant cushions, even lifting the rest of her body up slightly.

Twilight attempted to grab both sides of Rainbow's hips, but it was an impossible job now that they had become far wider than her shoulders. Twilight's balls began to churn heavily, and she knew she could only hold out so long with a body like this in front of her. She panted and groaned heavily, competing with Rainbow Dash’s constant moans and screams of pleasure. Feeling Rainbow's pussy tighten as she hit her orgasm was the last thing Twilight needed to finish herself. With a few more powerful thrusts she shoved her member deep inside Rainbow Dash's vaginal walls, releasing a tidal wave of seed.

Rainbow Dash could only lay there and moan in bliss as each wave of Twilight's cum was pumped into her rapidly filling womb. The rest of her body had finally stopped growing, it seemed, but now she found her belly plumping out slightly from the sheer amount of seed Twilight was releasing. Each throb of that two-foot stallion shaft prolonged her pleasure, continuing what had to be the longest and best orgasm of her life. She could be content spending the whole night just laying here on her beanbag sized breasts, being swollen larger and larger by Twilight's warm,thick cream. Maybe even getting hit with another one of those spells wouldn't hurt...

After releasing the last of her seed Twilight slumped down on top of Rainbow Dash's body, panting heavily. "I can't believe we did that," she admitted, "but I've never had such a great experience."

Rainbow Dash began to catch her breath as well, finally sorting her thoughts back together. "You know, I should be mad at you for how out-of-control that got....but I'll admit it. I had fun too; that was crazy awesome. You better be able to change me back though."

Twilight grunted as she worked to pull her huge shaft out from Rainbow's marehood and the crevice of her ass cheeks. "Don't worry, I will. First thing in the morning. I'm a bit spent on magic after all that."

Rainbow Dash rolled her eyes, "You just want more time to stare at my ass."

"That too," Twilight replied. She gave Dash's flank an affectionate spank and watched the tidal wave of flesh jiggle a few seconds before settling down. "Come on. Let's get you up on the couch and go to sleep. When we wake up I'll be able to reverse all this." With some effort Twilight managed to help Rainbow on to the nearby couch, although the combined weight of all Dash's curves was almost too much for the furniture to handle and she took up majority of the space. Twilight managed to get comfortable though, part of her body was resting on Rainbow's soft flank and she was able to use one of Dash's breasts as a pillow. They had almost fallen asleep when Rainbow Dash broke the silence.

"Hey, Twi?"

"Yeah?"

"Think we could do this again sometime?"