My family was poor when I grew up, so we never had video games, and my parents always cooked. My best friend's family at hot-dogs, cheese burgers, and pizza most nights of the week and sat around all day playing video games (it was lots of fun to go over there and hang out :-).
Anywho, they were all fat, except for one of my buddy's sisters, who took up jogging every day when we were in high school. I've known lots of fat/chubby people and they all live off of fast food and/or eat cheese and mayonnaise sandwiches every day for lunch. Of course there are a few people w/ disorders that make them fat, but that doesn't explain most of the population.