<EDIT> Picture didn't work, the graph is halfway down this page: Junkfood Science: JFS special report: Obesity statisticulation — When will people get it?
The dashed lines on this graph are the population distribution in 1976. The solid line is a modern one. "The obesity epidemic" is the result of a big chunk of the "norm" creeping over an arbitrary line, these people gain a few Kgs (which gives no significant increase in health risks) and all of a sudden we're going mental about the obesity crisis. The overall distribution is pretty similar and as you can see there were a ton of "obese" people back in 1976, contrary to popular belief. Also most of the health risks of being obese come at >40, and as you can see on this graph that much of the obese lie at less than 40.
The obese are a natural part of any well nourished population and always have been.
Americans have also increased in height by an inch over the last 40 years but their isn't a "height epidemic" going on is there?