why were people so skinny in the 70s?

So I'm trying to figure out why people were so skinny about 40 years ago vs today....here are some reasons i can think of and i want to know yours:

1. little to none high fructose corn syrup
2. more activity.....people didnt sit on their computers and smart phones all day

these are just two main ones i can think of, anyone else have any ideas?