Some possible reasons for the rise of obesity:
--As Cleveland noted, the rise of service sector jobs, coupled with the physical energy saving benefits of modern technology, have pulled Americans out of the agriculture sector. However, many Americans still retain a farmer's diet (i.e. foods rich in saturated fats and carbohydrates).
--Over the last twenty or so years, there has been a steady anti-smoking crusade in the United States. Anyone that has ever quit smoking knows how the pounds can pack on. In my case, I gained approximately 45 pounds seven years ago when I kicked the habit. Fortunately I shed much of that weight.
--Another possible reason is the increase of women in the workforce. With two parents in the workforce, there is often very little time for either parent to prepare a nutritious home cooked meal, so they will resort to fast food and quick, easy store bought substitutes.
--American car culture is partly to blame. Most American cities have expanded to accommodate the automobile. As a consequence, drivers don't expend much physical energy during their commutes (unless you count teeth gnashing, arm waving, and screaming). Most European cities and older East coast American metropolises, for instance, expanded to incredible size before the invention of the automobile. Even to this day, the most convenient mode of transportation in those cities is one's very own feet.