I know there are exercises you can do to make your bum bigger. I don't know that you're going to want thin legs with a badunkadunk though. You may end up looking like an apple on a straw, lol. Are there trainers at the gym you go to that you can ask the question of?
Here's a site I found that has the "Top Ten" of bum exercises, although it looks like most will give you some muscle in your thighs to match. Hope this helps!