Or do fat people move to the suburbs? Perhaps this is only a correlation - the causality might be that fat people need the wide open spaces of the suburbs to be confortable. They might move out there to be closer to their Crispy Cream donut shops and ginormous warehouse grocery stores...

The sidewalk issue might have a similar causal chain- fat people move to suburbs, fat people don't like to walk or ride bikes as much so fat people don't put in sidewalks.