I've been following a game called Project X: Love Potion by
zetar02 of Hentai-Foundry, which just released a new demo the other day.
It's basically a game featuring several of the female Sonic characters, along with some similarly-styled fan characters in a beat-'em-up playstyle against various enemies. Instead of taking damage, though, when the player gets caught by an enemy, they go through a small hentai scene, losing a life when they go through a certain number of these scenes and become pregnant.
This game is more straight-up hentai and cum-based belly-inflation/pregnancy, but in the second area, Chao are one of the types of enemies, and one of their animations involve them using an air pump to temporarily blow up the player's breasts. It's short but neat, and the rest of the game is pretty interesting too. One of Rouge's animations with the same level's Earth Worm enemy has slightly more subtle BE as well.
The site for the game is here, and if you're not interested in going through there, you can download the game directly through
Megaupload or
Rapidshare.