Excercise, my friend. Go and find a good gym or if you've got the money a good personal trainer. That should reduce the size of the buttocks a alot. Also consider if her buttocks is either mainly...
Any form of cardiovascular exercise combined with a sensible diet that causes a net caloric expenditure will cause a decrease in body fat, some of which would be on the buttocks. So yes, but so does...
Well, kinda. Short of liposuction there's no way to target where to lose weight/fat. But with the right diet walking will help you lose calories, which eventually will see your buttocks shrink a bit...