A muscle either grows or it doesn't... hate to break it to you.
So you have a few options:
1. Keep what you have.
2. Lose more fat and eventually some if will come from your butt... though it can be helluv hard for a female, especially as you get leaner. But losing the fat will a) make your butt smaller and b) make it more tone, possibly.
3. You can lift weights appropriately and coupled with the right mix of exercise and diet expect your muscles to grow a bit. Not much, but it could give your booty that 'pop'.
What's right?
Got me, that's up to you and your goals.