Er, on the contrary, I read that the obesity rate in America is actually decreasing now for some specific age groups, though I can't recall. Apparently the guy's results from Supersize Me were exaggerated - a later endeavour by someone else eating McDonald's for the same amount of time showed that he lost weight - due to exercise. (Sorry for being vague; I don't remember the specifics at the moment.)
While it's likely that portion sizes and nutrient content have changed from the time Supersize Me was done, it can also be pointed out that DPA does matter, not just the food you eat. Obviously eating a supersized McDonald's meal and then sitting at a desk for five hours straight every day will almost certainly cause weight gain, because you're taking in more energy than what is required to sit. However if you're a very active person (very very I guess since they are large portions), you might just put to good use all the thousands of calories provided in that one meal.
Point is, lack of exercise is also a contributor to obesity, not solely the food that is eaten. [/sigh]