What's up with my two friends?

Both are brothers, 29 years old.  They eat like shit (lots of fast food, snacks, soda, etc.) and get no exercise, have messed up sleep schedules, and pretty much do everything unhealthy.  They're probably both 5'7 and 145-155 at most, and out of shape.

One in particular is scrawny, and has a little "buddha belly", and occasionally looks bloated from time to time.

Most people who live like this would probably by morbidly obese, but these guys are the opposite....they're scrawny and sickly.  It's like reverse obesity.  I mean, they still have basically a lot of the same problems the obese have (more prone to sickness, etc.), but never seem to get physically larger (not by much, anyway).

Is there a term for this?

BBC Documentary: Why are skinny people not fat?

Part 1 of 6