Hungry Sumo has one, entertaining, original mechanic; and that's enough to carry a fast-playing little Flash game, though not enough to build anything bigger around.
Your sumos (pink-belted) and the opposing sumos (blue-belted in early levels) bounce around the window, rebounding off the sides and each other like billiard balls. Each time sumos collide, they both lose size -- the bigger sumo causes more of a loss than the smaller one. A sumo reduced to the smallest size that impacts an enemy switches sides.
As you mouseover a sumo, it eats rice, swelling in size; however, if it impacts an enemy while eating, it "pops," turning into a baby and switching sides. Consequently, you want to mouseover when there's no danger of impact and quickly move away at other times.