The only way to build a rounder butt is to do glute exercises with resistance. You have to actually build the muscle to make your butt look bigger....I had the same issue. I have been working with a trainer since January and this is what we have doing to perfect my behind
you need to have access to a gym
Take a weighted bar, or a barbell with about 10-15lbs to start....rest it on your shoulders. step forward and lunge 5x then step forward with the other leg and lunge 5x. then stand with legs shoulder width apart and do 5 squats, reapeat this sequence 10x.
on the smith machine put a 10lb weight on each side. Do 15 squats. Then switch to a lunge position and do 15 reps lunging on each side.
Use the glute machine and do 20 reps of kickbacks
take and exercise ball lay on the floor knees bend with your heels on top of the ball...slowly lift your glutes and back off the floor keeping your shoulders on the floor while squeezing your buttocks, do 15 reps
Leg press 15 reps start off with about 20 lbs
hip abduction and induction machines, three sets of 15 reps, start at about 50 lbs
as you do these exercises you will periodically have to add more weight as you become stronger, if you get through with your sets and don't feel the same burn as you used to then you know it's time to increase a little
my personal trainer