This is not just a southern issue, its a national issue. I agree with SC122's comment. Stay on course and don't worry about other people and if you do go out to eat reasearch the restaurant's website so you can make as wise as choice as possible.
As for the issue at hand there are many mitigating factors for obesity in this region, though not restricted to it:
Advertising; we are constantly bombarded with commercials, coupons, and specials to entice one to eat out.
Work envrionment; the place my wife works has rampant obesity due the sedentary nature of the job as well as the swarm of restuarants that deliver. Most employers don't condone or encourage healthy habits.
Retail enviroment; due to urban sprawl we drive everywhere.
Medical; Due to medical advances over the last two decades people unknowingly or knowingly have the false sense of security that doctors can cure it all. OTC medicine ads actually tell you its ok to abuse your body. Have you ever seen a heartburn ad featuring someone who got it from eating a nice healthy meal?
Social: Many feel its a part of aging. The most common and idiotic phrase uttered is "you're going to die anyway, may as well enjoy it." Anyone out there enjoy having a heart attack, diabetes, open heart surgery? Being 'big' is almost revered as a sign of strength where as being short and/or thin is regarded as a weakness (mostly with men).
Financial & convenience misconceptions; People believe that it is cheaper & more convenient to eat out. Not always the case.
Just my .02
