Also, Rachel had the advantage of being a former athlete, so there's the muscle memory to consider, and even though she was very overweight she wasn't very large for an extended period of time. Genetics, body type, factor in as well.
Regarding compression garments. I read an article recently that says people don't use them properly - they should be worn after exercise to get maximum benefit. I have noticed BL contestants wearing them during workouts, but I assumed that was for comfort - jiggling skin would be at least uncomfortable, if not downright painful, while running or doing pyro. Makes me think about women wearing Spanx (a woman I worked with wore it EVERY day, all day) and if it's maybe not a great idea to wear it for vanity.