Dancer's dents? Why does my butt look so flat on the sides? (Photo)

I'm 20 years old. 5'6" and 95 pounds. I eat just fine and have always been really thin which I don't mind. What bothers me is that despite having somewhat toned perky hips and a tiny waist, my butt looks square directly from the back. Why? Not sure if that procedure of inserting fat from the thighs into the hips would be deemed appropriate for me since I'm already so skinny. Squats and lunges have been a help but I'm convinced they mainly build my thighs! I just want a more feminine round butt.