While a lot depends on your genetics, if you want a smaller butt, lose fat while avoiding heavy weights on lower body exercises. Cardio and a good diet will help.
Unfortunately you can't really do spot reduction on your body - when you lose weight, you lose it from all over your body and not just a certain area. That said, lunges are very good for your butt and thighs.
Squats and lunges are excellent exercises to tone the muscles in the buttocks as well as the hips and thighs. If you hold light weights while you are doing them, they are even more effective.
