That extra insulation around your butt does not grow because of any kind of exercise; it's genetic and from eating too much. Don't worry about any exercise making your butt larger--you couldn't even measure the difference. Just keep moving, eat less and watch it shrink. If the exercise is done at the correct intensity to make it aerobic, the spur will not be sufficient to cause muscle hypertrophy (growth). Often what happens is that when one begins an exercise program, their hunger may increase or they may begin justifying additional food – the "I deserved this" syndrome. If caloric consumption exceeds expenditure, then fat stores can increase. This may occur on the thighs and buttocks.