Soarin can grow bigger by drinking his own cum. Rainbow Dash is in a bedroom with him as he autofellates himself in his sleep. A bigger Soarin naturally has bigger balls, so his growth spurts get bigger as he does.

Briefly, Rainbow Dash wondered what Soarin thought he was sucking on.

The male Wonderbolt shifted in his sleep, mumbling something unintelligible as another line of drool spilled out of his mouth. On any other night, that saliva would’ve gone to wet the piillow, but tonight, Dash had pushed his back against the headboard of their bed, positioning his hips just the right way for his lips to naturally brush against his cock’s flared head. The moment his muzzle had touched his stallionhood, he’d latched on, and had been sucking ever since.

Rainbow Dash grinned to herself as she saw Soarin’ shudder with his orgasm. His first, she expected. She knew just what his cum did to him, and she wasn’t about to let this be anywhere near the last time he tasted it.

Even in his sleep, Soarin’ moaned quietly, his hips thrusting up gently into his mouth by pure reflex alone. His cock jerked and twitched, his muzzle up and bobbing down in an irregular fashion as more and more of Soarin’s cumshot blasted right to the back of his throat. Dash would’ve cackled with glee if she wasn’t afraid it might wake him up. Instead, she was satisfied to run one hoof down toward her nethers. She began to rub softly, moaning in time with Soarin’.

As his orgasm finished - she could hear him gulping down his own cum, with only a few drizzles of semens dripping from the corners of his muzzle - Dash again wondered what he thought he was sucking. She wondered idly if it was her. She’d been meaning to try out that spell of Twilight’s, anyway. Maybe tomorrow -

But no; Soarin’ had barely finished sucking himself dry than the effects of his cum took off. Dash watched greedily, still rubbing her clit as Soarin’s cock began to lengthen. It was only an inch or two, but that was more than enough to his entire head up into his muzzle. His balls, each one the size of a small orange, swelled up too, not only refilling with cum but also growing even further past their original size.

Soarin’ had always been “big” for a stallion, but it hadn’t been until a week or so ago that Dash had really learned why. In a way, she was amazed at his self contorl. If he was only this big… Well, it was easy to think that if she’d been in his place, she wouldn’t ever have settled for “top five percent.” From the way it seemed tonight would go, neither she nor Soarin’ would have to settle ever again.

His muzzle pushed further down- hungrily, Dash would’ve said - down his cock until he was taking a full six inches of his fat, throbbing stallionhood in his mouth. She would have been surprised if it hadn’t already been filling his jaw entirely, but still he managed to suck greedily on it like she’d never seen a pony suck before.

Soarin’ groaned and drooled around his boner, by now just under two feet long and certainly in the upper two percent of pony boners. Dash just got more excited and wet watching him. She wondered how it tasted, how it felt filling up his mouth and groaning with a need to spill the seed from his full, churning balls.

And then, almost before she knew it, he was cumming again. This time, it was a bit more difficult to notice - his growth had pushed his cockslit almost all the way toward the back of his throat, and so it was all the easier for him to gulp down more and more of his seed. She watched with awe as a bulge swelled around the base of his cock, travelled upward through his shaft, and finally burst his in his mouth in a flood of hot stallion seed., His cheeks and neck actually bulged out slightly from the sheer volume of cum inside of them. But still, Soarin’ gulped on, drinking down his own cum until it looked like he had a slight potbelly. She wondered at the sheer amount of cum inside, sloshing around his belly with the barest motion of his body.

Just like before, it took only moments for his seed to do its work. Rainbow Dash’s hoof sped up in its masturbatory ministrations, and at the same instant that Soarin’s cock twitched with the promise of further growth, her legs convulsed around her hoof. She bit down on a cry of pleasure, keeping herself in check as she came. A spurt of sweet, sticky marecum flooded out from her pussy to stain the sheets. She moaned softly as she eyed the much larger puddle of stallion cum that Soarin had let drip down the side of his enormous shaft so far tonight. Dash panted, rubbed her puffy, hot clit a few more times, and prepared herself for a show.

This time, Soarin’s growth held absolutely no subtlety. His cock merely throbbed once before swelling wider and longer before Dash’s eyes. His balls sloshed with refilling cum, actually pushing his legs as they expanded to the size of grapefruits.

Soarin’s cock pushed further into his muzzle as it grew; for his part, he was content to faintly gurgle and slurp around his enormous head as it pushed past his tonsils and into the top of his esophagus. Halfway down, his medial ring bulged and swelled until it was rubbing against his chest. So long and hard was his cock that his body actually started to move forward, the force of his growing cock against the back of his throat enough to push his hips away from the head of the bed, which naturally pulled his head down as well.

On and on he bloated, his shaft and balls growing until there was surely no stallion in Equestria who could surpass them in size. And yet, Rainbow Dash only found herself growing more and more excited.

Bigger balls meant more cum. And more cum meant a bigger Soarin’. Eyes glittering and hooves dripping with her own juices, she reached out and began to stroke Soarin’s throbbing erection. Soon, his snorelike slurps and her own long, measured strokes were the only sounds that filled the room.