Alot of them put on weight right after leaving the show which is understandable as they prob do alot less exercise/eat more and hydrate more than they do on the show.
Ive seen the season 1 winner,I rememeber them having him come onto season 9 to talk to the contestants about how he'd put back on all the weight and more afterwards (it was either him or erik from season3..not sure which!). I havent seen season 1 but aparently he was a big gameplayer and Ive always thought those that go for the game and money are more likely to fail in the long run as opposed tothose who work out why they became that weight and identify their problem.