I think this is because of my impression, and an extremely busty 24 year old cutie theorized this herself, that there are essentially two different weight gain patterns among women. There are women who tend to add fat to their butts, and hips, and women who tend to add fat to their breasts and bellies. Now, obviously this is grossly oversimplifying things, but it does seem that, in general, women with disproportionately large breasts tend to have bellies, whereas women with big butts tend to have big hips as well.