Squat is well-known exercises to get bigger butt. for Squat :

1.Stand up straight with your feet about 1 foot apart.
2.Slowly end down from the knees, aiming to keep your back straight and never letting your kness extend past your feet. When you go down, squeeze your butt so it firms it. Look straight ahead and squeeze your bum while bending your knees.
3.Go down as far as you can comfortably go.
4.Slowly rise back up to your starting pOSition
5.Repeat 10-20 times
6.Take a break 

Swimming, especially legs-only (kick) with a flutterboard will definitely work your butt. Check out the butts on Olympic swimmers.

To build a bigger butt, you might need to include squats and sprints to your list of exercises.

The active butt exercise.
•Get on your palms and knees on the floor.
•Raise one leg back and up, keeping your knee bent.
•The movement is from the hip joint while the back and the waist remain still.
•Visualize pushing your foot up towards the ceiling but without extending the leg.
Alternate for 20 repetitions on each leg without a break. Sit back on your folded legs and relax for 30 seconds.
Make another 2 sets of 20 repetitions on each leg.

Lunges also work to firm up your butt. If you are not currently physically fit, you may have difficulty getting down low enough for maximum effectiveness. Don't worry, start doing them as low as you are capable of and work you way up to doing deep lunges. Once you have mastered them, you can hold weights while lunging to increase your workout.