What timing too because around this time of year is when most people especially moms start putting on winter holiday weight just because.
When it comes to being overweight, I think most of it is because of ignorance and laziness. People say they "work out" or are on a healthy diet but probably don't have the experience, lifestyle, or knowledge on how to get and maintain results. And even if they do know better it's still really easy to balloon up like after graduating a boot camp when you don't have instructors and prepared meals forced on you
Being "lazy", or not working out every other day, has absolutely nothing to do with being overweight. Plenty of people work out every day and are still overweight, at the same time, plenty of people do not work out ever and are thin/look great.
Our ancestors certainly didn't lift weights or run marathons every day. They took it easy and relaxed, conserved energy, until they needed to hunt or gather. Even while hunting it was only short sprints that were the extent of strenuous exercise.
I think humans always hated those who couldn't carry their own weight so to speak. For what it's worth the so-called ideal human form has been seen to at least the ancient Greeks, who were probably also much more active in fat shaming than society today.
Our hunter-gatherer ancestors used their own body weight and had a pretty high activity level, we don't really see too many obese natives unless they trade for McDonald's. In tribal societies it probably doesn't pay to get flabby while a rival is out there hunting