My mom was thin in the 70s but she was always drinking Tab. No doubt all the sugar has had an effect. I would also question the diet drinks, too. They all play with metabolisms in my view.

You could also add that families were more united in the past. Communities probably were too. People stayed married despite problems within their marriages. Most diets were a lot more local. Less fast food and foreign food. Less gym memberships I would think too. A lot of things were different.