Elizabeth Kolbert paraphrasing Brian Wansink:
[People] 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 the French-fry eaters.
The entire essay – a digest of recent literature on why obesity is increasing in American and around the world – is excellent.