I think a lot of it is fairly obvious: diet and exercise. My evidence is pretty simple: look at cultures (including local cultures) where everyone drives everywhere, versus cultures where everyone walks everywhere. You just don't see morbidly obese people in cultures where they're forced to walk a lot; it's mainly an American and Western phenomenon (it's worst in America and Mexico). Diet is probably another big factor: in obese cultures, calories are cheap, but the food quality is generally very poor.