People are more overweight these days because they eat 250 note calories per day than people in 1970: http://wholehealthsource.blogspot.com/2008/12/us-weight-life.... Increased caloric intake explains almost all of the rise in obesity: http://www.sciencedaily.com/releases/2009/05/090508045321.ht.... Why do we eat more than in 1970? For the same reason we buy cars more often, phones more often, have several TV's in each household: we're a consumer culture and we have more resources to consume.

If you want to look for the root cause, look at food businesses and advertising. The Starbucks' of the world that make more profits than the coffee shops of yore by selling higher calorie foods for even higher prices. A coffee and donut at DD has half the calories of a latte and scone at Starbucks.