I think there's a series of very complex and intimately intertwined issues at play here. Europeans eat differently and they take more public transportation than the average American (larger US cities are typically more active/fitter largely for this reason), but they also have the huge benefit of a welfare state system where certain things are significantly more accessible because a publicly supported choice was made in this regard.
1. School cafeterias are supplied differently - more fresh food, more variety. Here's a fun fluffy overview:
http://www.attn.com/stories/6085/school ... ted-states2. Universal health coverage allows for more frequent doctor's visits, letting medical professional catch things like diabetes or PCOS faster and manage them more effectively, including through healthier diets; this is particularly important for lower-income individuals, and we've seen time and again in peer-reviewed studies that obesity is still very much linked to poverty for large segments of the population.
3. Universal health coverage + labor regulations + the ensemble of family/social support measures in place allow for more meals cooked at home, more vigilance over family health, and thus more attention to what is eaten and how
4. US restaurant portions are out of control. I live in a big city with a lot of chi-chi food offerings, which are supposed to serve smaller portions, and I've taken to just ordering off the appetizer menu because it's already a meal and a half. Took me years to realize this. A really nice lunch place by my work sells fantastic four-cheese mac for $3.95 as a side for their sandwich menu; that mac is itself a full dinner.
That's just a part of it. I do think it's an important question because it really speaks to how different societies are structured and what's important to them, and we could all use more cross-cultural understanding.