Here's the deal: Yes, your metabolism affects your size, and the faster your personal motor runs, the more calories you'll burn. But the truth is, a slow metabolism isn't the real reason you're packing on pounds. You are the reason.
As a general rule, arrive at the airport no later than two hours before your domestic flight is scheduled to depart. This is somewhat later than the typical suggestion of three hours for an international flight.
I believe the rule is to be there two hours before your flight. The assure you that airport security will remind you of this and have no sympathy for you if you're about to miss your flight. I speak from a friend's first-hand experience.