Sitting down all day doesn't just make it more likely that you'll gain weight. It makes it more likely that you'll gain weight on your butt, specifically.
The body produces fat when more calories go in than out. But it also produces fat when that area of the body is stretched for long periods of time, according to a study recently published in the "American Journal of Physiology -- Cell Physiology," and reported by the Daily Mail. When a body part consistently bears a big load, it seems it builds its own cushion.