Big Macintosh hurriedly closed the barn door, slamming it shut and locking it. Quickly, he peered around the room, making sure all the windows were shut and that nopony was looking through them. To further console himself, he started blocking them by stacking objects in front of the windows.
"Darling, I don't think anypony's going to be looking in on us," a fine voice cooed. Rarity was laying on a hay bale in the middle of the room, her tail raised in a needy manner as she waited for him to come to her. "Besides, it's the middle of the season, everypony will forgive us if they catch us."
"Nuh-uh," Big Mac said, "Mah sister's here, and Ah don't wanna haf'ta explain what we're gonna do to her." He continued stacking things in front of the windows until he was fully satisfied that nopony could watch them.
Rarity stretched out, shaking her rear invitingly to him. Her face was almost red, with sweat running down her forehead. "Hurry up already, darling, Momma's getting a little hot over here."
Big Macintosh grinned stupidly, eyeballing her rear as his dick came out of his sheath. Trotting forward, his dick waved around between his legs, his balls giving off a musky odor as he approached her. Rarity rolled off of the hay bale and stuck her behind up, blushing as he came closer. "Uh," he said, "Ah just wanna say thanks for-"
"Oh, spare me," Rarity whined, "Just get on with it, big boy." Without any hesitation, Big Mac climbed onto the white mare, and pushed his erection into her pussy. "Erk! Oooh..." She squealed, her libido spiking up as he entered her, "You really are big!"
Big Mac's hips went forward, pushing his cock deeper into Rarity's pussy, her juices spilling over his shaft. "Nnrgh," he grunted, humping her hard. Back and forth, he kept pushing his dick in and out of her.
Rarity made little high-pitched squeaking noises, feeling his massive cock move around inside of her. "Ooh, oooh!" she moaned, rocking her hips to match his rhythms, "Deeper! Go deeper!"
He thrusted hard, his balls swelling and churning as he went further into her soft, moist pussy. Precum leaked out of the tip, running down his cock as he rammed her.
Suddenly, his dick began tingling, and he felt Rarity get tighter around his member. "W-whuh?" he groaned, slowing down in his humping. His balls were heavier, and swelling up much more than normal, while his dick was now going deeper into her by itself.
"Oooh, w-what are you doing?" Rarity moaned, riding his dick like no tomorrow, "Don't slow down!"
--
"Are you sure this is the right spell, Apple Bloom?" Scootaloo asked, peering into the spell book, "Eddy doesn't seem to be getting bigger." She looked at the strange images of oddly shaped sticks on the page.
"Of course it's the right spell," Apple Bloom replied, holding a rooster in her hooves, "Sweetie Belle's just not castin' right."
Sweetie panted, blushing. It wasn't enough that she had been feeling weird lately, now she was exhausted from trying to cast this one spell. "I think I'm doing it right, Apple Bloom..." she whined, her horn fizzling.
"How are we supposed to get our cockfighting cutie marks with such a small cock?" Apple Bloom said, holding up Eddy, "Now come on, try the spell some more, sixth time's the charm!"
Sighing, Sweetie Belle lit her horn up, and prepared to cast it some more.
-
"Mah dick's growin'!" Big Mac yelled, bucking his hips as his already huge cock swelled up inside Rarity. His legs were beginning to push apart from his already watermelon-sized balls, and his dick was pushing itself harder into the mare.
"Oh, keep growing!" Rarity moaned, loudly, "Fill me to the brim!" Her stomach was bulging out from his cock, and her pussy was almost torn from his sheer girth.
Unable to resist himself, Big Mac kept humping, his once footlong dick now rapidly expanding to massive size. Rarity felt good to Big Mac. So good, in fact, that he didn't even notice himself raising off the ground as his huge balls extended past his hooves. She felt so tight, so small against his massive dick.
"Oh Celestia!" she screamed, her body essentially becoming a cock sock for Big Mac, "Oh, oh!" Her backhooves flailed wildly against his massive shaft, while she twitched around in pure bliss.
A voice in the back of Big Macintosh's head was worried about how big he'd become, and if Rarity would be alright, but he couldn't stop himself. Bigger and bigger he got, his dick now twice the size of his body, while his balls churned with unreleased seed. There was a massive flash outside, and his dick grew even faster, from ten feet, to fifteen, to twenty. "B-bigger!" he moaned, his hooves wildly running up and down his dick as it grew. Rarity barely fit on his tip, screaming and panting as his massive cock stuffed her. Harder and harder he pumped, running his hooves all over his dick, until finally, he came.
Rarity's stomach bloated, quickly being filled with gallons of his seed while huge globs of cum sprayed down out of her pussy. She couldn't believe how much cum he was making, her already bulging stomach was rapidly swelling as his dick hosed her with semen. She managed to wriggle off after a moment, but his cum-blasting didn't end. He sprayed everywhere, painting huge splotches on the walls and ceiling white, plastering Rarity's entire body with a thick coat, seeping into her mane and tail. Finally, with one last huge thrust, he was empty, and his massive dick went flaccid, easing Rarity to the ground and letting her roll off.
"Oh dear," Rarity giggled, waddling up to him as he rested on his huge dick, "If I had known you could do this, I would've asked a lot sooner." Sitting on her haunches, she relaxed and patted her largely distended stomach, cum oozing out of her vagina and sliding down her torso. "Mmm, I hope Sweetie Belle isn't home, I'd hate to explain this to her."
Big Mac just sighed, contently laying on his huge cock. He was pretty sure he wouldn't be able to move for a while, but he was just fine with that.
--
"H-hey, Sweetie Belle?" Apple Bloom asked, poking her friend's unconsious body, "Y-y'all alright?"
Sweetie groaned, putting a hoof on her head. "Yeah, I'm fine..." she grumbled, leaning up. Abruptly, Apple Bloom made a loud gasp, and Scootaloo began snickering, her face blushing. "What's so funny?" she asked.
"Uh, Sweetie Belle," Apple Bloom said, making an uncomfortable smile, "Ah, uh, Ah think ya were righ'. That wuz th' wrong spell..."
Sweetie raised an eyebrow, then turned to Scootaloo, who in turn, just kept giggling, pointing downwards. "What? Is there something on my face?" she asked, then looked down. Between her hind legs, a cock of her own was growing, a little flared tip showing as it grew outward.
"AAAAAAAAAAAAIIIIIIIIIIIIIEEEEEEEEEE!"
Author's Note
To put it shortly, I wanted to write a porn. It's short, and half the fetishes aren't even mine, but hey, if it helps some folks' dongs expand, then it means that I've done a good job.