How do you get a big butt by just eating?

Naturally sitting on your bum will make it bigger. The body likes to cushion itself, so if it is constantly sitting on a chair, at the computer then it will grow custom to it and provide extra cushioning (the body likes to make its self comfortable.) But chances are if you sit on the computer for too long and not get out once and a while you might find your self gaining a bit of weight.