I think that the problem is a strange mix geographical that cultural.
Take for example, people who live in "food islands" compared to those who do not.
The food island people have very few choices, which are generally fast food or the dollar store. These food is all high in calories and carbs, very low in other nutrition, and unsatisfactory to eat. Even if this person eats the same as someone who buys all of her food from Whole Foods, the islander is going to be taking in much larger amounts of calories and, if you consider the dynamic interactions of food and the body, likely doesn't have enough other nutrients to offset or or properly incubate what little nutrition they are eating.
For example, compare the off-brand cereal you'd get from the dollar store and compare it to the off-brand you'd get from a reputable grocer. The stuff from the dollar store shows nothing but zeros all the way down the box. The off-brand from the grocer shows quite a few items.
Next, consider the type of food you can get from different areas. You can't find this thing in Los Angeles:
And good luck finding a buffet. There are a few, but nothing like other areas of the country. In Clevelend, there was at least 5 different buffet companies and lots of one-off restaurants and Chinese buffets around. Eating at a Chinese restaurant that wasn't a buffet was an absurd notion. Florida and even San Diego was similar. All of these places had plenty of fat people.
The size of the average dish in the South was huge. I could easily buy one dish and take it home and eat it for two to three days.
Ever seen a Po' Boy?