I would not say eating out several times a week is daft, more that in the UK it is prohibitively expensive.

We eat less
drink more
laugh more
don't take ourselves so seriously
don't angst about things
have a national health system
and all the other things Cotwold Scouser said.

I think exercise is definitely a factor. Americans are so dependent on their cars, it's absurd (yet complaint today about gas prices - try going for a walk for once!). Of course, I am the first to complain about people these days taking the train into town because they are taking my parking spots! Anyway, back to the topic, I think that the amount of processed junk we eat here is also part of the problem. Trans fats are so much a part of the typical American fare. Check out what people put into their shopping carts at the grocery store. I am always amazed at the lack of plain old fruits, vegetables, meats, and dairy items in there. Seems like people pile in the processed packaged cookies (rather than making their own with sugar, butter, eggs and whatever else you put in there), pre-made microwave frozen dinners, frozen veggies drowned in a nasty processed cheese sauce, and my favorite, the vacuum packed meal in a box. All of those things have way more fat, sodium, preservatives and god knows what else in there than anybody needs.

The theory about people outside of cities being fatter seems to be true, too. Having recently moved to the burbs I've noticed how little people walk. In fact, there was an article in our little town paper about people complaining of the lack of parking spaces in our town. The reality is, you can always find a spot here within 3 blocks of wherever you want to go. People expect to be able to park in front of store A, then get in the car to drive 2 blocks to store B, etc. People who work in the city, though, seem to get a bit of walking in and it shows.