Symbolically Fat is Tolerance

 A thick protective suit between you and the outside world, see? A blobby buffer.

 Every time you put up with defection, you gain a layer of fat.
 Social poison. Also physical poisons, like car exhaust and glyphosate. Gets stored in the blubber. Especially when you exceed your steady-state tolerance limit.

 If you stop putting up with the bullshit, one way or another you will slim down. 

 Being fat is low class because being a doormat is low class.
 Americans are especially fat because they especially love treachery and getting victimized. Their lab rats are especially fat because they're especially poorly treated. Their cars are fat because they're designed and built by abusive Communists (but I repeat myself).

 Note that along with poisons, the fat stores hormones. When you lose buffer fat, you will re-live the emotions of the poisonous time. Also also psychic poisons. Fatness is an ill-health debt, a delta between earned harshness and experienced harshness. The debt has to be paid back.