I hope this is an acceptable environment to give this answer........

I used to be CEO of a company that did some work with Mayo Clinic (Rochester) a few years ago. We worked specifically in the weight loss field.

Although I can't locate the Mayo reference material, I can tell you that, calorie wise, American's aren't really consuming that many more calories today than they did in 1970. Yes, I know that may be hard to believe....but it's true.

However, in 1970, we weighed ~ 24 lbs less per person as nation as we did today.

Mayo's conclusion was that about 10% of our weight gain as a nation stemmed from a slight increase in calorie intake and the types of foods we were eating.

The other ~ 90% was a result of our sedentary lifestyles. According to Mayo's research, we as a nation "moved" about 8 1/2 minutes more per hour back then than we do today.

By "moving" they mean just getting up and walking around, standing or doing something other than just sitting. By moving 8 1/2 minutes less per hour, we decreased our NEAT Calorie burn (NEAT = Non-Exercise Activity Thermogenesis) by around 150 - 300 calories per day. The amount of NEAT calories (or any calories) you burn is dependent on your age, body size, sex, and a few other factors.

So by moving 8 1/2 minutes less per waking hour (say 16 waking hours in a day), we are essentially sedentary an additional 136 minutes (2 hours and 16 minutes) per day. That is a stunning amount of time!

Do that for several decades and VIOLA', you have a nation that has gained an average of 24 extra pounds per person.

Read Warren Belasco's Appetite for Change! More later, but it actually discusses food politics in this time; there is also an unpublished dissertation by Gabriella Petrick out of University of Delaware addressing postwar/ late-twentieth century food habits that can answer your question. The basic gist is that food was getting increasingly processed, but fad diets were also multiplying. I do think your question is as much about bodies as it is food, so I'd remind you of the postwar abundance in the 1950s; everyone could buy as much food as they wanted! Soon, it became a badge of honor not to consume as much as possible, but to consume properly (see Jesse Berrett, “Feeding the Organization Man: Diet and Masculinity in Postwar America,” Journal of Social History 30, no. 4 (July 1, 1997): 805–25.) Self-control of consumption soon became strong and attractive and skinny to an extreme became the rage - google search "Twiggy photos" for an example of the ideal. Skinniness became a way of bucking the dominant culture of mindless consumption - see skinny early David Bowie.

(I do wish I could give a more detailed answer and perhaps I will expand this tomorrow, but I have been drinking, and don't want to start speculating too wildly.)