The legs are the biggest muscle group in the body. By working these muscles you will burn more calories. Butt crunches will build a firmer buttocks. You will lose fat as you do them.
Will Butt Crunches Make Your Butt Bigger Or Smaller?
Most likely just firm it up.
Smaller and tighter if you're overweight; however, a bit more pronounced and obviously firmer if you're thin.