In 1970 Americans already had an abundance of calories available, but they didn't yet have an obesity epidemic. One big difference is that in 1970 a typical person could afford 4000 calories of food a day, but most people still ate food cooked or assembled from basic ingredients at home. It's possible but less likely for someone to prepare and eat 4000 calories a day worth of homemade fried chicken, cookies, mashed potatoes, etc.

Americans today can afford to eat 4000 calories worth of food and it's already optimized for palatability and convenience. It's relatively easy to eat 4000 calories of Doritos, microwave burritos, and boxed cookies. There's advertising to remind you of its existence and researchers dedicated to optimizing the delight of eating these products (increasing the odds of overeating just because it's pleasurable and frictionless).

The transition from "abundance" to "abundance multiplied by advertising and product optimization" drove obesity more than simple availability of calories, IMO. I see a parallel with digital information. There was more than enough information on the Web to spend all day looking at it even before social networks were common. But that "home cooked" experience wasn't engineered for engagement time, so companies that optimized products for engagement were, in practice, a lot better at getting people to look at digital information for many hours per day.

People also smoked cigarettes in the 70s

Cigarettes are an appetite suppressant. Easier to control your calorie intake if you aren't feeling hunger pangs often