There were certain concessions one had to make when one had tits as big as Derpy’s. Certain doors that were too small for her to fit through, places she just plain ol’ couldn’t go. Physical limitations that she couldn’t overcome without cutting down on the boobmass – which was simply unacceptable. It was just a fact of life that her colossal sweater puppies took up more space than most entire ponies. Each. Though “sweater puppies” was a bit of a misnomer because she mostly just wore bikinis. That was another reality of her boobs; they leaked. A lot. The slightest stimulation ran the risk of setting them off, and if another pony got involved it ran the risk of them getting bigger as well. And so Derpy had to avoid tight, crowded spaces as much as was possible.

Or at least that was the case in theory. But, well, Derpy was only equine, just like everyone else. And, just like everyone else, she had her kinks. And the big one was her boobs being too big to fit in a space that she was in anyway.

To her credit, she tried not to engage in that particular kink too often. It was a bit rude to the various passerby and random strangers that it inherently required. And, while Derpy was certainly a very assertive and forward pony, she wasn’t rude. But, well, sometimes she just needed that very specific kind of stimulation. Nothing else would do; she needed a physical, tangible reminder that she was fucking huge. And what better way to get that physical, tangible reminder than to go into a space that was just plain ol’ too fucking small for her and just fucking existing.

The first step was to pick a place. Somewhere with limited space and lots of people. An exceptionally busy shopping mall was a good one, but Ponyville wasn’t super close to any malls so it wasn’t convenient and anyway she couldn’t predict how busy any given store would be on any given day. Which was important because this wasn’t exactly something she could engage in on a daily basis. Or even a weekly basis. So she preferred a more consistent venue, which is why this time she’d decided on a train. Normally that was just the end of it. She bought a ticket, got on a train, and then just waited until she was satisfied she’d been enough of a nuisance to get off a bit. But sometimes even that wasn’t enough for her needs. Sometimes she needed to be a bit more... proactive.

This was one of those times.

The train was not particularly crowded, but that was fine. It was a Saturday morning, it had all day to get crowded and she didn’t plan on leaving any time soon. The good news was that it was exactly crowded enough that she could justify electing to stand rather than sit, and also that there were a bunch of people who had elected to sit rather than stand. Derpy manoeuvred her way through the train, putting in exactly enough effort into dodging other ponies that she didn’t quite succeed but it looked like she was trying her best, before settling on a victim.

Ugh, she really didn’t like using that word. But, unfortunately, it was the right word. There was no denying that she was kind of the asshole in this situation. But that was something she could feel guilty about later. Right now she was more focused on the pony she’d decided to make use of. They had to be sitting, and obviously she’d prefer someone attractive. She’d decided on an athletic looking pegasus with bluish green fur and a yellow mane. She was buff but not too buff. Which Derpy could see because, much like herself, the pegasus mare wasn’t wearing much in the way of clothing. Just a sports bra and some daisy dukes that were just tight enough to dig in slightly to surprisingly ample hips and ass and tits. Not even comparable to Derpy’s massive milkers, of course, but impressive in their own right. Were they not juxtaposed against a literal freak of nature Derpy might even go so far as to call them huge.

And, as a nice little bonus, there was a sizable bulge in the crotch of her daisy dukes, too. Not likely to come into play, but there was always the chance she might turn out to enjoy this and want to come home with her for some more.

Derpy quickly settled into position, looming over the smaller mare – whoever she was looked to be fairly tall, but generally speaking in any given comparison there was going to be more of Derpy than there was of any given other pony short of Princess Celestia herself. There was a moment of silence while the pegasus continued to focus on her phone, not quite noticing Derpy at first. Eventually a frown spread across her face and she looked up. “Excuse me, could you-” whatever words the stranger had dried up as she made eye-contact with Derpy’s mountains.

“Sorry,” Derpy said with feigned innocence. “Not enough space to sit.” That was a lie – there were still plenty of seats free. If the other pony pressed her on that front she’d feign concern for any potential elderly or handicapped ponies who might get on in the near future. “I can move, if you want?” This was her extending an Out. If the other pony elected to have Derpy stay in their personal space, it was tantamount to consent. Or at least that’s what helped her sleep at night.

The other pony’s frown deepened. She didn’t look angry, per se, though. More... conflicted, especially with the way her face had flushed slightly. “... Fine, whatever, it’s almost my stop.”

“Thank you!” Derpy replied, before settling back into silence for now. The other mare continued to frown, but sort of returned her attention to her phone. Sort of, because she very obviously kept stealing glances at Derpy’s scantily clad tits. That was partially an intended effect on Derpy’s part – she liked the sensation of eyes on her body. But also it was another necessity of her anatomy. She was a growing girl, after all, and she could only afford to buy bikinis that fit so often.

The train lurched to life, the inertia setting her tits wobbling slightly. With a smirk, Derpy leaned forward ever so slightly, leaving even less clearance between the stranger’s face and her chest. “Um,” the turquoise mare said, her voice tinged with annoyance and also something else. “A little breathing room, please?”

“Hm? Did you say something?” Of course, that was a lie. Derpy knew exactly what the stranger had said.

“... Never mind.” The stranger’s slight blush turned outright furious as she tried to look anywhere besides Derpy’s cleavage – a difficult task considering it was inches away from engulfing her head. Swaying gently, back and forth. Almost hypnotically. She swallowed heavily, leaning back in the hard plastic seat in a futile attempt to gain some more breathing room when faced with the enormity of Derpy’s tits. She had, at this point, abandoned any pretense of fiddling with her phone, and after a moment of what was probably thought, she slid slightly in her seat in an equally futile attempt to reach her bag, which sat on the floor in between the two mares.

It quickly became clear to her that that wasn’t gonna work, or at least not without her leaving her seat entirely. No, if she wanted her bag, she was going to have to lean forward.

Which was a problem because Derpy was in the way.

“Um, could you please-”

“No.”

“Oh.” Pretense had been abandoned, it seemed. “It’s just-”

“I know. You can reach it without me moving.”

“Yeah, but I don’t want to-”

“Yes you do,” Derpy interrupted again. “Go ahead, I don’t mind.”

“... Right.” The stranger swallowed heavily – at this point Derpy couldn’t really get a good look at her face, but she could hear her swallowing before carefully leaning down and forward in an attempt to get at her bag without burying her face in Derpy’s boobs.

An attempt that was also futile, as Derpy picked that exact moment to pretend there’d been a bump in the tracks and collapse forward on top of her. “Oh, my,” she said. “Somepony’s being assertive. You could have just asked, dear~”

“I’m- I...” the smaller mare stuttered slightly. “Oh Celestia they’re so big.”

“And they’re all yours for now~”

“It, um. It really is almost my stop, ma’am.”

“I know. I just don’t care. And I don’t think you do either.” Derpy leaned in closer, reaching down to grab the stranger’s hands and slip them under her bikini. “If you really want to leave, I won’t stop you. But I don’t think you do.”

“... No, I don’t,” the stranger muttered before leaning deeper into Derpy’s cleavage and beginning to squeeze her nipples.

“Good girl,” Derpy cooed. “Get momma Derpy nice an’ warmed up before mealtime.”

“M-mealtime? Wait, you don’t-”

She didn’t need to finish her sentence. The stimulation from her nimble, athletic hands and the way her body was pressing into them was enough stimulation to get a good trickle going. Of course, a trickle by Derpy standards was gallons by normal pony standards. And this was just the beginning.

“What’s your name, dear?”

“It-it’s Lightning Dust, ma’am.”

“It’s nice to meet you, Dusty. I’m Derpy Hooves, but you can call me Mommy.”

Dust swallowed heavily. “Y-yes, mommy.”

“Now,” Derpy said, licking her lips eagerly as she leaned back just enough to let out Dust’s head. Still gently holding the smaller mare’s hands, Derpy used them to move her bikini top out of the way and unleash leaky nipples onto the world. “Drink up and grow nice and big and strong for momma.”

“I- I- oooooh fuuuuuuuck~” Dust groaned in response. There was a loud tearing sound as the zipper of her daisy dukes failed miserably and something big and long and thick emerged, sliding in between Derpy’s boobs and out the top. It was a darker green than Dust’s fur, and drooled a thick stream of cloudy precum that was very nearly a match for Derpy’s own milk output in terms of volume.

And, as Derpy stared at it, it slowly pulsed bigger.

The smaller pegasus let out a low groan, no longer capable of coherent sentences. Her body slipped into autopilot, reflexively thrusting her hips to titfuck the massive mounds of flesh her cock was wedged between. “Aaaaahngggg~” she moaned, in conjunction with a spurt of precum.

“Don’t worry, baby,” Derpy cooed gently as she leaned into the massive leaky flare, licking her lips hungrily. “You just drink up, momma’s gonna kiss it and make it aaaaall better~”

With that, all of the buildup was over. Derpy took Dust’s shaft into her mouth and began titfucking in earnest, while Dust quickly did the same to one of Derpy’s teats and suckled eagerly.

That was enough stimulation and enough intimacy for Derpy’s condition to kick in. There was a tightness. A warmth. A fullness as her production began to outstrip her output and how much storage she had and her boobs began to grow. And grow. And grow.

And they weren’t the only things that grew. When Derpy had told Dust to drink up and grow big and strong, that hadn’t been entirely euphemistic. Ever since the... incident with Twilight, her milk had gotten significantly more nutritious than it already was. Everything a growing filly needed to grow big and strong. And if you weren’t growing anymore, you would. The effects were sadly not permanent, but usually they lasted just long enough to have some fun.

Dust’s body let out a low creaking noise as her bones lengthened and thickened from all that calcium. There was a low rubbery stretching sound as she got taller, and as more flesh blossomed into existence. Her already ample chest quickly began to strain against her sports bra as her own milk production started to kick in. Her hips widened and her ass swelled and her muscles went from defined to bulging.

And, most pointedly, her balls swelled even more rapidly than Derpy’s tits. They went from cantaloupes to the size of her head and then bigger with no sign of stopping as their cum production kicked into high gear. Fat and fertile and aching for release into the nearest fertile mare.

Which Derpy was perfectly fine with. She was probably going to have to pay to replace the train car once they outgrew it, but she could pretty easily afford that at this point – her milk sold for a lot, it turned out. And anyway, it had been a while since she’d gotten a good breeding. She kind of liked the idea of a foal or two or ten. Or more. Lots more.

After all, there was more than enough milk to go around.

Author's Note

Patreon reward for Nimnul again! This one stars the same Derpy as was in Feed the World.