This came from
http://www.medpagetoday.com/PrimaryCare/Obesity/tb/14945.
I don't know about all the criteria used.
I think the reason people in big cities are thinner is they do a lot of walking from place to place whereas in smaller cities- where things are more spread out- people drive from place to place.
I've seen many people drive around and around in parking lots looking for a close to the store parking space rather then parking out further and walking a little extra distance.