The U.S. adult population in the 1970s ate an average of 2400 calories a day. By the 2000s, our calories were up to 2900.
Putting a face on the statistics
What did average adult weight do as we increased daily calories by 500? It increased by 8.6 kg, from 72.2 to 80.6 kg. In U.S. units, that’s a 19 lb gain, from 159 to 178 lb.
Children increased their average intake by 350 cals/day over the same time frame.
If I recall correctly, I’ve seen other research suggesting the daily calorie consumption increase has been more like 150 to 350 per day (lower end for women, higher for men). I suspect these latter figures are more accurate.
Details are in the American Journal of Clinical Nutrition.
The study authors don’t say for sure why we’re eating more, but offhand mention an “obesogenic food environment.” They don’t think decreased physical activity is the cause of our weight gain; we’re fatter because we eat too much.
h/t Ivor Goodbody
Worth saying this is disputed but…simple explanation of obesity epidemic may be we eat 500 cals more than in 1970s http://t.co/k3RQnIdCGn
— Ivor Goodbody (@IvorGoodbody) May 22, 2014
Paleo Diabetic 
At least a part of it is due to too much omega-6 which stimulates us to overeat. Here is a reference to an article on that:Lipids. 2014 Jan;49(1):59-69. doi: 10.1007/s11745-013-3842-y. Epub 2013 Oct 1.
Dietary linoleic acid elevates the endocannabinoids 2-AG and anandamide and promotes weight gain in mice fed a low fat diet.
Alvheim AR1, Torstensen BE, Lin YH, Lillefosse HH, Lock EJ, Madsen L, Frøyland L, Hibbeln JR, Malde MK.
Thanks for that link. I’d like to see the human study, if it exists.
-Steve
I haven’t seen a human study, but to a certain extent, I am one. I tried to lose weight for over 20 years and slowly gained over 20 pounds. When I found out about all the effects that omega-6 was having on me, I went in a diet to avoid omega-6. In 3 weeks I lost 15 pounds (not from calories–from fluid). I also was able to stop taking blood pressure, arthritis, heart rhythm, and pain meds in that 3 weeks. I had been on Indocin for 35 years. Since that time (May 2013) I have lost 35 more pounds at 2 to 3 pounds a month. I no longer feel the need to get full before I quit eating. I promise, I had really tried for 20 years, now I am normal weight for the first time in my adult life.