There's another, related explanation, which is superstimuli. A lot of modern processed food is developed to maximize its taste impact. If there's a feedback between loop between good taste and higher consumption, widepsread and rising obesity may be the result of processed foods breaking this feedback by tasting too good. "Low-fat" may also exacerbate the problem, since fat helps signal satiety, so high-sugar, low-fat foods provide calories without curbing hunger much.

A genetic or evolutionary explanation seems unlikely because obesity is distributed non-randomly throughout society. It affects certain regions, subcultures, and classes very differently. This could be partly explained by the tendency (via affluence, culture, whatever) of certain groups to avoid eating highly processed high-calorie foods. It may be explained simply by net calorie consumption, but I'd be surprised if that itself is not correlated with the consumption of high-calorie processed foods.