I have several friends, who are weight lifters. They are not bodybuilders and none of them have big butts. Why because they wear high heeled boots for lifting. They get a great deal more support for their backs and legs wearing high heeled boots during weight lifting. One of them suggested to just raise your heels appx. 1-1/2 inches of the floor when you do squats. He thought that would do the trick, that is, keeping a flat butt. He laughed and said. " get a 2 x 4 about 2-1/2 to 3 feet long and plant your heels on it during squats".