You are an idiot. They might be flexible but that is still bodyfat you are seeing. We all know how subcutaneous fat appears and moves. They more than likely possess more lean muscle under that layer of fat thereby also increasing size/appearance.
Let me guess, you probably also believe that when a muscular person stops lifting, that it "all just turns to fat". As opposed to what typically happens is that actually, said individual does add bodyfat and lose muscle, but also retains more muscle mass along with added bodyfat thereby giving the impression of being bigger.
I think you will also find that it is a matter of muscle and force propulsion, especially in the upper body. It is a fact that even very fat athletes such as powerlifters...though fat....carry just as much muscle if not more than in lighter weight classes. Ever notice too that women in sports such as softball are very typically carrying extra bodyfat? Women cannot put on muscle mass as easily as men, fact, naturally they don't have the hormones to. However the added bodyfat does allow them more weight and leverage ,and possibly more muscle mass retention. But definitely allows them more mass and power expression too which they otherwise can't achieve from just muscle as easily as men. Just as a heavyweight powerlifter seeks to do.
And lastly, just as many female runners and bodybuilders many times encounter, too low of bodyfat throws hormones so out of whack that many females stop having their periods.
The points regarding cold and insulation and calorie ant hunger signals above are very valid too. Happens with male swimmers too, they just have more lean muscle to burn more of the calories off.