There are perhaps 3 factors in question 1) lifestyle--fast food, increase in consumption of food away from home, decreased physical activity, occupation 2) agricultural subsidies and 3) growth in agricultural productivity (smaller % of household food budget, global phenomenon).  Studies are not definitive.  Economic simulations have found that removing subsidies for grains and oilseeds would have negligible effects on caloric consumption and eating habits (Rickard B. Health Econ. 22: 316-339 (2013). There are also a number of other plausible contributors--sleep deprivation, pharmaceutical-induced weight gain (see this comprehensive review article).  This is a complex question and is probably multifactorial with the BIG 2 most targeted at this time.  (