You claim this, but now if you goto a department store, you'll find the mannequins are all a little bit heavy, as in, overweight.

If you go into department stores, the vast majority of mannequins aren't clinically overweight unless you are specifically shopping in a plus size store.

Even if this were true, your argument doesn't follow from the evidence. The existence of clothing for overweight people doesn't imply that people believe that being overweight is healthy. It only implies that overweight people exist, and someone wants to make money selling clothes to them.

Duck 'vanity sizing' sometime.

Google 'average mannequin size'. The average female mannequin is underweight, not overweight. One study did a survey and found that 100% of the female mannequins they examined represented a clinically underweight woman.

Vanity sizing is a thing because female sizing completely arbitrary and stores want to make women feel better about their weight. If anything this is evidence that fat acceptance hasn't caught on--if it had, people wouldn't be ashamed to wear a larger size.

Regardless, I'll say it again--the fact that overweight people exist says nothing about the assertion that many people believe that being overweight is healthy.

1. https://jeatdisord.biomedcentral.com/articles/10.1186/s40337... This 2017 study did a survey and found that 100% of the female mannequins they examined represented an underweight woman.