If suburban sprawl causes obesity why are New Yorkers fat, too?

Many scholars are looking for a link between the walkability of sprawling areas to high rates of obesity. So far the evidence doesn't support the idea that sprawl makes people fat. In New York City, which is about as far from suburban as you can get, people are fat, too. So much so that the city government has banned trans-fats and requires calorie counts for all menu items at fast food restaurants.
But those efforts aren't enough, and now the city is spreading the work among the straphangers. The Department of Health is placing advertisements on the subway to reduce overeating (over-ordering to be more precise). There is no doubt that the city is walkable, transit oriented and has a low rate of driving. These factors have been in place for decades. Yet New Yorkers are fatter just like the rest of the country, and the city seems to think it has more to do with portion sizes and what we are eating than a lack of active living.