Hmm, I expect the whole "why Americans are fatter" is unknown really, but I'd agree that they eat more dairy and more meat. Portion sizes are generally wildly bigger than here. (Whenever I go home, and eat out, I end up massively over ordering for the first few days, and ending up with tables covered in food )
North Americans are also, generally, much less active than Brits. They can be, the whole place is set up for sloth. There are plenty of suburbs where walking and biking just aren't options. (Granted, there's a percentage of them who are really fit and healthy and super aware of what they eat.)
I think the consumerist culture, very cheap fast food, and the high-fructose corn syrup in everything are part of the problem, too.
So yeah, lots of reasons. They do stick to white carb (and organic food etc is much less available there than here, ime ... well, I'm basing that on Canada, actually, but still ...), but I suspect the carb thing has little to do with it all ...