Why were we thinner in the 70s?

Why did people seem thinner in the 1970s? Simpler diets, more active lifestyles, less screen time, and less stress: the lifestyle of the time largely explains this difference. Some habits from the past could even inspire us today.
We often look at old photos from the 70s with a certain surprise: the figures seem slimmer, more natural, as if everyone was in good shape without following any particular diet. So naturally, one question often arises: why were people thinner back then? Was it the food, the lifestyle, or simply an impression? In reality, several habits of the time explain this difference, and some could even inspire us today.