When did fat shaming as we know begin? And how do you figure out your true weight?

This is really a bunch of questions revolving around health/fitness/fat shaming. I’m just writing something up and want to research some things.

I assume the whole “going to the gym” thing started around the 70’s, but I just wonder how and why. When did it develop into “skinny bikini chicks” as “the norm” for guys to be interested in? I mean, it wasn’t always like that since humankind started, right? Being fat was treated differently ages ago, just a little bit. It wasn’t as fanatic or “Oh my god you have love handles you need to stop eating right now” sort of thing.

Also, from a physics standpoint, your weight is different on other planets depending on its gravitational pull. So what does that mean for how much you TRULY weigh and how do you really know when there’s an issue? I’ve often heard that BMI is a poor indicator of weight, although I want to research its flaws.