Well, clearly the contestants were at the far end of the weight spectrum to begin with, but regaining weight after an intensive, successful weight loss programme isn't rare, even for people who were only mildly to moderately overweight. That suggests that their problems aren't unique to their particular metabolisms, just more extreme.
Also, their pre-competition metabolic rate was normal for their size, and after their weight loss it cratered, and never properly recovered. Even if they had pre-existing physical or psychological abnormalities that contributed to their heaviness, this is a clear, objective measurement that started normal and has become different, simply through losing weight.