It all depends on you genetic make up. Do you gain weight in your thighs and ass? Weighted squats,steppers and other movements can make them larger.
I would stick with some weightless movements such as free hand squats,walking lunges, step ups on a bench.
Work up to 100 reps. Vary your foot position on the squats such as a wide stance,meduim,toes pointed in or out. Use big steps and get deep with the walking lunges. Do them outside so you can walk 100yrds. Step ups should be on a bench about 24 inches off the ground. 3 sets of each should do the job.
Workout everyday alternating movements.
Remember diet. You can have a firm butt but hidden under fat.
MM