I think Jason used to be fat but since he started to date with Britney he got in better shape (Ryan Seacrest said that in an interview with Britney and Jason during the Femme Fatale era).
It was the opposite for Justin. He got hotter after he broke up with Britney.
The ones she actually likes (ie Justin) dont gain any whilst the sex slaves she keeps in her basement to fill the hole that is missing in her life. You know, cuz ****ing will.i.am would get annoying after 2 minutes.
Probably because the pressure of being Britney Spears's boyfriend got to them and they wanted to whip themselves into shape. Once they split from Brit, people weren't following them around as much anymore so they let their exercising habits leave them.
Jason 