This is something I definitely believe.
The availability of affordable healthy food in urban areas is a lot higher.
So are bike trails, walkable spaces, etc.
There are studies I've seen that show that when an area gets things like bike/walking trails etc. there is an uptick in fitness participation.
WI definitely has plenty of obesity; it also has lots of cross country ski trails, hiking/biking trails etc. and that's fairly state wide.
Diet is a huge part of limiting weight gain but I'm amazed when I go back to my ruralish home town and hardly anyone walks or runs for the benefits. Several summers ago I was at the local bar which is just over a mile of mainly gravel road from my house and people looked at me like I was crazy for walking home.