You might enjoy Vale City! The character sprite “stages” are easily controlled through fullness, and two of the three player-controlled characters have a focus on belly expansion, as well as 1/3 of the supporting characters.
It’s a notoriously “grindy” game, so exploits have been developed and have been shared in the thread here on the forum. Let me us know if you find any titles you like in your search, I’d also love to hear about games I haven’t played 