Do high heels make a girl's bum look smaller?

Wearing high heels should not effect the way your bum appears. What can effect the way your bum appears is the type of panties you wear, and the type of jeans you wear. Obviously, the type of jeans you wear are going to change the way your bum appears. Tight jeans will make your bum appear more shapely and tight. Wearing saggy jeans will make your bum appear saggy and unshapely.

High heels do, however, effect your posture, and are not necessarily good for your back.