the part we control is probably smaller than most people think

Think it would be more accurate to say

the part we control is probably different from what most people think

The idea that body type is basically determined by genetics simply doesn't work, because current body types diverge rather dramatically from body types of a hundred or even fifty years ago. Genes don't change that fast.

It's more likely that people are responding to culture and incentives that are largely beyond their control. Culture and incentives can change dramatically in the necessary time frame. This doesn't mean that people can't make the right decisions, so much as it suggests the right decisions may be hard to figure out. Choosing to associate with people who will look down on you for super-sizing your bacon double cheeseburger may do a lot more than deciding to get a diet coke with it, but good luck selling a diet book centered on that idea.