I just listened to a recent Scientific American podcast about this. The link is, um.... here:
http://www.sciam.com/podcast/podcast...95&ref=p_sciam
One of the biggest culprits is how industry is producing about 3900 calories per capita per day: more than almost anyone needs. The result is that we look for more places to sell these calories. It has now become socially acceptable to eat anywhere - in the car, on the walk to work, and even while working. A couple of decades ago that didn't happen as much. Remember when cafes in bookstores were novel (pardon the pun)? Well now it's expected.
The podcast also brings up the fact that we intake more calories through our beverages.