If you really want to own a great-looking butt, you should have "no buts!" No buts --- no excuses, no lousy justifications, no petty whinings, and no puny rationalizations. If you want it badly, work for it! Sweat it out! Exercise! Design a program and follow it religiously.
The normal misconception of most people is that you can have a nice round butt by simply wearing a tight jeans. Sorry to disappoint you guys, but the clothes you wear only highlights what you have, it cannot ALWAYS totally hide what you lack. But do not worry, gaining a good tight and round butt is too much easier than adding bustline (for women) and expanding your chest (for men). How come?
Two reasons: 1st) The butt, all things being equal and proportioned, is basically the part where we can find the most number of fats in our body. So in the case of flat-chested women, if they feel the need to enlarge their breasts, they have to add fats on it (but since the process is complicated and risky, silicon implants are popularly practiced). 2nd) The butt is part of our lower body and we use our lower body more rigidly everyday than our upper body. Our lower body supports our upper body in standing, balancing, walking, jogging, running, and cycling.
This is why we should have a stronger lower body than the upper body, stronger legs than arms, or else having the opposite, we should be walking on our arms! Try arm-standing for 2 minutes, and see if you can live with that everyday. So the key factor here is work your lower body!