I was a cereal addict as a kid. Killed a couple boxes of the stuff a week. And I was super skinny, probably because we played outside to the point of exhaustion every single day.
The problem is that habits formed as kids die hard. You keep eating the same way as you did when you were 12, but have nowhere to put it all, except your belly / thighs / butt. Before you know it you're 27 and huge.