Jin (and pretty much every other JE idol out there) is not fat.
I use Jin cuz he's the most frequently criticized and has mentioned trying to lose weight on tv, but he's hardly the only case. Half of k8 think they are gigantic for some reason.
And yes, I realize that "norm" in Japan is smaller that say, the US. But really, Jin did not come back from LA morbidly obese. Yes, he looked bigger than the rest of KAT-TUN, but that's cuz he is. He's the "strong one with the broad shoulders/back." Besides, the rest of KAT-TUN are teeeeeny. They have small frames and they're on the thin side. Jin's got a pretty face, but he's still built like a dude. And he's like average weight MAX.
I don't know about the rest of you, but I'd much rather have a guy that had more meat to him than someone so thin I'd be concerned about him cracking a few ribs if you hugged him too hard. I remember seeing Kame in Tokyuu Tanaka 3 Go! looking a lot more solid, and he looked good, but he seems to have lost it again. Pity.