I want to improve my butt size! I want it bigger, how can I do that?

Maybe exercise. Leg exercises could enlarge your butt but why bother. A large butt is as sensible an attractive feature as big feet. Exercising and eating right for fitness and learning to love your own unique and special features is more important than enlarging your butt, feet, breast, ears or anything.
Squats and lunges. The butt is made of the muscle, gluteus maximus. Exercises that strengthen that muscle may make your butt get bigger. But honestly, i urge you to do these exercises for the health benefits (improved strength, endurance, function, and metabolism) and not for the supposed aesthetics. Get comfortable with the way your body is made and its unique size/shape. I'm sure it fits you already.
Exercise protocol. There are alot of exercises that can substantially improve/increase a person's buttocks size. It would probably take too long to list all the specific exercises here but if you're interested contact me and i can submit a list of exercises.