why were people so skinny in the 70s?

I think part of the answer was demonstrated in the documentary Super Size Me with Morgan Spurlock showing how portion size has increased using McDonalds and 7-11 as an example. The current children's hamburger and small french fries was considered sufficient for an adult. A Big Mac really was quite an increase in serving size. Same with the history of the Big Gulp which I recall was too much for a teenager in the late 1970s. In our family we found a 12oz can sufficent. No microwave and minimal snacks at home meant no nervous eating, no oral fixation that needed to be constantly fed.

We ate snacks-potato chips, candy bars, cookies but packaging appeared much smaller than now. I notice the huge size of potato chip bags and candy bars today. I think about one pop-tart was sufficient for one day's worth of snacking.

Also we kids were poor and relied on parents for foodstuffs and they were rather discerning of how much of our intake should be quality food vs. snacks/junk.

Then as was mentioned no cable tv or computer meant you were outside engaged in activities. But we did watch plenty of televsion. There was much programming for kids and one did watch quite a bit from 8pm thru 10pm M-F-Sat-Sun. It was family time to be together.(Does that last sound rather quaint?)

We did eat home cooked meals then. Reasonably healthy by today's standards. We were kids and so portion size was small and grew as we grew. Active mid-late teens are consumers! A Hungry Man tv dinner from Swanson was a rare treat for us. No one ever liked the cherry cobbler dessert in our household I recall. Given to the dog.

Eating out at KFC or Pizza Hut was a rare occasion. Portion size was also smaller. Adults smoked quite a bit which I understand is an appetite suppresant. Folks drank more as well I believe as social drinking was drinking and did not involve eating wings or cheese fries,etc. Perhaps peanuts is at most what you saw in a bar. I recall having a small bowl of homemade dip with ritz crackers was a big treat for the Superbowl not a massive spread at all compared to what is done now in tailgate parties.

We had one classmate out of 220-odd group who was obese in HS. I recall because he could not do one push-up or one situp for the annual Presidential Physical Fitness Test. I do notice the majority of young folk today are overweight. We just did not carry around that much weight in my city during our youth of the late 1970s-1980s.

I was born in 75. I was in elementary in the early 80's and we came home from school around 3:30, did any homework we had, and by 4 or 4:30 we were outside until dinnertime (at 6) and if there was any daylight left we were back outside again. Catching frogs, playing kickball, riding our bikes. I had an Atari growing up but that was mostly once it was too dark to be outside or when the weather was bad.