Idk what to put here
i dont know alot of 3D weight gain games
but heres a good one
magical seed garden
Not sure if you’re into anthros, but there’s Kobold Kare. Might be a tad more inflation than weight gain, but it’s the best I’ve got right now. Alternatively, you could check out the user-maintained list of games.
its a post here
note : its 64 i think