I hear that most people who are able to eat like sh*t and not gain weight are of the A positive blood type. Are you A positive?
In any case it'll catch up with you, I don't believe doctors when they say the ability to eat sh*tty foods and not gain weight is a product of a "good" matabolisim. I have a friend who eats like crap all the time but he gets gas more than anyone I've ever met in my entire life, which is a strong sign of poor digestion. So my guess is that alot of those callories are simply being passed through waste insted of being stored.
I'm sure your short on muscle though, as most poor eaters who don't gain weight are. I'd say that looks worse than someone with a little extra weight on them.
I can't eat junk food, not even rice or breads unless I'm doing some exercise right after, or else I gain weight. I weigh 210 to 215 on average and have a huge amount of muscle mass compared to your average person that maintains itself with little effort on my part. My dad was the same way, so there HAS to be a genetic link to body type.