Originally posted by Dead Man Flying 
Get in my belly!
-- Todd/Leviathn 
Hilarious! 
 Seriously, though, Beet1e brings up a couple of good points and leaves out a few:
(a) without fail, television commercials between 9 and 10pm will feature pizza, burgers, and tacos.  The reason?  The food industry knows that most people eat dinner around 7pm and will get hungry 2 to 3 hours later.  They _want_ viewers to make that late-night snack run through the drive-thru window.
(b) running, as a sport and hobby, is in the minority.  The premier professional sport in America is American football, which favors athletes carrying a lot of physical bulk and running short distances (very few runs are farther than 8 yards).
(c) the second-largest professional sport?  NASCAR racing and other forms of motorsport.  Big calorie burners there.   
 (d) Americans only: Quick! Name a fast food that doesn't involve burgers, chicken, cheese, butter, or being prepared in a fryer.  Now, when was the last time you ate there?  How many times do you eat there in a given work week?
(e) American men see bulk as a sign of physical dominance - the bigger you are, the more manly you are.  Its a social thing.  Its no secret that American male business suit sizing is designed to allow for greater physical bulk than European cuts.
Those are just a few points to consider.  I'm sure I'll think of more.