I honestly think its because they drive everywhere and live in suburbia. A lot of parts of Europe have cities and rural areas, but not really 'suburbia' as much as in the US. Its easier to lose weight when you live in a city and walk everywhere, or live in the country and work on the land, but most people who move to the suburbs gain weight because their movement becomes limited to "to the car" and "from the car". Of course that's just my opinion!