I was talking to my editor, when she mentioned that she had an Astrophysicist friend. (This is just one of the reasons I am not an editor. My friends fall into the "let's go to lunch" variety.) He offered her a simple explanation of Einstein's theory of relativity: E=MC2.
If I understand her, it means that when a lot of pressure is applied to an object, it becomes heavier and then something else, that I can't remember, happens. ( I never did listen in science.)
It doesn't take much of a leap to realize that Americans are under lots of pressure, so that is why they are becoming heavier. Blame it on physics and have another cheeseburger!