Public Transportation
Living in Detroit, where all transportation is via automobile, I'd have to say being overweight is due to a lack of public transportation.
I lived in Vienna for a semester, and lost about 30 lbs. during that time. I wasn't starving myself either - mmm... Viennese food. Lotsa meat, tons of chocolate, lard spread on bread, many many litres of beer, and ooh -- Italian Ice!
No -- the big difference was that I walked everywhere. I figure I probably averaged 2-3 miles/weekday, and weekends when I visited other European cities I would probably walk 8-10 miles/day on average. If anything, those numbers are on the low side. Total: 20 miles/week easily
In Detroit, I walk from the front door to the driveway, from the parking lot to my cubicle, where I sit all day, and then reverse the process to get home. If I'm feeling especially energetic, I'll take my daughter for a walk or to the zoo. Total: 3-4 miles per week probably.
So... to get the same amount of exercise I got just living in Vienna, I'd have to budget at least an hour/day -- more on the weekends -- just to exercise, which I personally find boring.
No wonder I gained that 30 lbs + right back.