When I purchased this, I debated whether I should get a small or medium. (I'm 5'4", 130 lbs) Since I do not like clothes that feel too big, I went with the small, and it was fine. The elastic in the waist and the top allow it to fit a number of waist and bust sizes. There was plenty of room in the seat for me; however, I have a rather flat rear end. I would suggest that someone with a large, full buttocks buy the larger size.