I think you're vastly overestimating how many calories are burned walking and how many calories are in the average fast food meal. I saw a stat breakdown on the average meal at mcdonalds and it showed that someone could literally walk every minute of the day that wasn't spent eating and still gain weight if living on fast food.  Sure, someone can be in great shape or even healthy eating at a fast food place every day. I'll always defend burgers, which I think get a bad rap as being unhealthy. But no fat person is going in to get something like one plain burger and water. I mean a large fry alone is 500 calories and the best you can hope for with fat people is that would be the entirety of the side rather than just a portion of it.