Sorry, South Beach diet, it’s over. You were just an aberration for the Fast Food Nation.

Sales of burgers, fries, and doughnuts are soaring, according to Business Week. Fried chicken’s selling so well that KFC’s even thinking about spelling out its name again.

Americans are back on the junk food bandwagon in denial or defiance of all the warnings about high fat diets. But are the fast food outlets really to blame?

Morgan Spurlock morphed in one month from lean and vital to lethargic and doughy with his all-McDonald’s-all-the-time-diet in “Super Size Me.”

Now comes “Portion Size Me,” a documentary from James Painter, professor at the School of Family and Consumer Sciences at Eastern Illinois University, who makes the case that the true culprit is oversized portions.

A new study from Cornell University backs up Professor Painter’s claims. Researchers found that “large portions of food push people to overeat—even to overeat foods they don’t like.”

In the study, moviegoers were given medium and large buckets of stale popcorn. Those with the big buckets ate 34% more than participants with medium buckets, even though the popcorn was two weeks old.

Evidently, if you serve it, they will eat. And eat. And eat.