It's very possible that there is an external influence that's causing people to feel the need to eat more and gain more weight from what they're eating - this is precisely my point, and the point of the article.

The fact that you can force yourself to eat less than you feel the need to and lose weight in the process does not prove, as you seem to think, that weight gain is simply explained by people being weak willed and eating burgers instead of salad. You still have to ask why this change actually occurred, since it seems that it has occurred in the general US population about 50 years ago and most of the world has been following suit.