You are right. I've been visting the South every year for pretty much my entire life (most of my family lives in Alabama and Georgia). During my childhood, almost everybody down there was rail thin and heavily tanned (weatherbeaten). Nowadays, I dare say that the obesity problem is more pronounced down there than it is even up north.
I was just down there last week and I noticed a lot of obesity (something I pay more attention to now that I'm losing weight myself). By my reckoning, the biggest contributing factor is air conditioning and satellite TV. When I was a child, nobody had air conditioning and the nearest TV station was in Birmingham and it hardly came in at all. So we always played outside. No matter how hot it was outside, sitting under a shade tree was always cooler than sitting inside. These days, most of my family down there sits inside all day watching the 300 channels of TV that is now available by satellite.\
I brought my son with me on this trip and he had a hard time getting anybody to come outside and play with him. They just don't do that much anymore.
Also, I should mention that I did 6-8 mile walks every morning and I didn't see a single soul. All I got was a bunch of dogs chasing after me!