[H]e has performed all sorts of experiments to test how much people will eat under varying circumstances. These have convinced him that people are—to put it politely—rather dim. They have no idea how much they want to eat or, once they have eaten, how much they’ve consumed. Instead, they rely on external cues, like portion size, to tell them when to stop. The result is that as French-fry bags get bigger, so, too, do French-fry eaters.
From an interesting review of some of the literature on obesity by Elizabeth Kolbert.