I'm not sure if anyone has already found this, but there's an inflation manga called "Balloon Fight (Pregnant Looks Maiden Battle)". The gist of it is that certain girls are picked by teachers for an inflation-centric tournament. To prepare for it, they are anally fed a saline solution that expands them until they look pregnant. Then, two contestants thus inflated have to sumo wrestle until one loses her bowels (and the fluids). It's better than it sounds, and even includes a cat-girl and bunny-girl for extra points!
The manga is here: http://g.e-hentai.org/g/69248/e8a1238e91/ (I was surprised myself when I found this.) Enjoy!
You know, only Japanese can do such things.