I think there are a few reasons why we are fatter today.
For starters, I think suburban sprawl is a big contributing factor. It is my impression that kids and people in general in the 50/60/70's just did a lot more walking throughout the day. You would walk to the store, more kids would walk to school, you would walk to go see a movie. Etc. With the expansion of suburbia we are using our cars to get places to more than ever. At no time when I was growing up was it possible for me to be able to walk to anywhere fun. I couldn't walk to a park, or a movie, etc. I had to get rides.
This is probably also why in the south, where some areas have a mile between your neighbors, that there could be a contributing factor to obsiety. In the northeast there are still a lot of small colonial towns where you can get around really easily. In western washington (king county, my old stomping ground), the area is practically made for bikers, with wide bike lanes on most streets.
Obviously video games, more tv, and the internet probably also contribute to obsiety. Face it, there are just more things to do these days while sitting on your ass. We now have a whole generation of children raised on world of warcraft, aim, and 200 channels of programming.
And yes, lastly, poor people go to mcdonalds more.