Here's some conjecture: The people that were driven to be healthy in the past were about as healthy as the health-obsessed people today. And the people in the past who didn't care defaulted into a lifestyle and eating habits that, based on what was marketed and available to them, left them looking and being average from a weight and health context.

Now, they've similarly "defaulted" into the lifestyle and eating habits that are available to them, but those are worse for their health.

I think historically people were thin due to malnourishment.