That actually flattens the butt and makes it look smaller, kinda like how a corset works on the waist. :/
It's hard to find the right jeans - if it fits just right on the butt, it's too loose on the waist and the legs; if it fits right on the waist and the legs, it's too tight on the butt.
I used to buy what fitted my butt and just let it hang on my hips, but it was baggy on the legs, specifically the knees, and looked sloppy - not to mention how it made me look bigger than I really was because of all the extra fabric hanging around. Now I just buy stretchy jeans that actually fit my waist.
If the jeans are to tight or small it sucks in your butt in my opinion... I used to wear tight jeans and when I wore jeans a little loose people would tell me my but was big even though I feel it isn't they kept saying that "my tight ass jeans were hiding my butt"