It happened rather slowly.
Sonic encouraged him to sit down and eat something once in a while with him and Tails. Then, he started taking him out for ice cream after their races, insisting that three scoops was necessary to get all the flavor.
Sonic always looked so happy to see him happy, and he usually found food that Shadow liked, so there wasn’t any problem.
That is, until he was showering one day and noticed that when he set his arms down they sank a bit into his stomach. Not much, it was just a bit of a potbelly, but he could hook a finger under the bottom and jiggle it a little.
He wore a sweater to work the next day, claiming he was cold, and when Sonic called asking if he wanted to come over, Shadow wondered if it was he’d done this intentionally.
Well, he could play this game.
Tails wasn’t there, pulling an all-nighter at his workshop, leaving just Sonic, Shadow, and a whole lot of pasta.
Shadow ate one plate while Sonic chattered on about Eggman’s robot of the day, then stood up to scoop himself another. Sonic got a little quieter, but continued to talk as Shadow slurped up forkful after forkful of the heavy noodles. Sauce dripped unto his sweater, giving him a good excuse to tug it off and give Sonic a good view of his bulging midsection as he finished his second plate.
He set a hand on it, waiting to see if Sonic would laugh and tell him to put the sweater back on.
Nothing. He squeezed his legs together as best he could, popping up his belly from where the very bottom had rested on his chair.
“Ugh... do you want leftovers, or can I have one more?”
Sonic snatched up his plate and refilled it, setting it within easy reach as Shadow swallowed, sticking his fork in and getting to work.
The last plate went down hard- a single helping would have been enough, but he was on his third of the carbohydrate-heavy meal that settled in his gut like a rock. He managed to get halfway through before his complaining stomach called it quits, gurgling enough to be heard half the house away.
“You can’t waste it...” Sonic said, almost in awe, and he picked up Shadow’s fork with a few noodles still speared on it.
“If you’re su-” Before he could finish, Sonic stuck the noodles in Shadow’s mouth, fork pressed so far back it almost invoked his gag reflex. Shadow barely got a chance to breathe before his mouth was stuffed again, and he moved his hands to his stomach, massaging it as Sonic forced more and more down his throat, each bite harder to swallow down than the last until tears prickled at his eyes and the plate was empty.
“Holy shit, you actually did it.” Sonic set down the fork, eyes wide at Shadow’s heaving stomach. Every breath he took was difficult, and it showed. “I was gonna have ice cream, but-”
“Next time.” Shadow finished.
“I take it you figured out I like this? I mean... what a show there, Shads.” Sonic’s face was crimson.
“I wondered, and figured I’d-” Shadow burped, cheeks flushing. “...Test it out.”
“And it looks like you like it too...” Sonic’s tone slipped to teasing as he leaned over, running his fingers over Shadow’s mostly flat chest and down to where it met his ballooning stomach, tracing a vertical line that curved along the other hedgehog’s food baby- all the way down to where his legs met in a growing erection.
“Hhhhhh, maybe... just a little...”
“You like being stuffed up, like a prize pig...”
Sonic made his fingers into a little clip around Shadow’s lips, squeezing them shut.
“You like being fat, I like making you fat. I think we can work something out.”
At first, it was just once a week. Sonic would either call in take-out or make something, always getting a little bit more than needed, a little bit more than before.
Then, twice a week. Shadow made the excuse that he’d been getting more days off. Sonic never really had to work at all, aside from being called in to help with whatever villain needed dealing with.
Three times a week. After their weekly sessions, he hadn’t gained much, but when it was bumped up to two, then three...
Shadow always started by himself, but by the end, Sonic was slipping bits of chicken or spoonfuls of ice cream into his mouth as Shadow forced out belches to free up that extra bit of room. Often, Sonic was straddled over Shadow, until one week his legs didn’t quite fit.
“Holy...” Sonic caressed his now-boyfriend’s stomach, still empty but wobbly and now too big for him to get his legs around.
“What, you don’t like it?” Shadow grinned, leaning forward enough to set his finger under Sonic’s chin, making him look up at him. “I thought that was the whole point of all this.”
“Y-yeah, but I honestly thought you’d stop me before this point.” Sonic grabbed a handful of flab, shaking it and sending ripples all around. “I mean... look at this! You’re so fat, you could probably be a sumo wrestler and still have enough left to make a whole new person!”
“And whose fault is that?” Shadow said, fangs still glinting. “This-” he slapped his belly, and Sonic could feel it through the fat in his hand- “Is both of ours. You want to take care of it? Because I think I just heard the doorbell ring, and you said tonight we’re having enough chinese to fill the entire table.” He pulled Sonic close. “And I want all of it in this big, fat gut.”
Sonic decided it would be a good idea to put on some pants before answering the door, and Shadow decided that he liked this whole thing very much.