When such a large portion of the population is overweight it tends to indicate a systemic problem rather than one of individual choice.

My point here though is that it’s only a minority who “can’t help it, they were born that way”. Most would be able to overcome this issue if the dynamics were different.

I see the body positivity movement as just trying to reconcile that reality. If you are already obese then you might as well try and find a way to live a happy life because you are probably never going to be slender again.

This is a weird trope. Most models are selected to be slender so they can, basically, simulate a clothes rack. No curves to draw attention!

I'm not sure the fashion industry is telling anyone this is ideal. I can't remember, in my lifetime, when skinny models were ever really looked at by society-at-large as the "right" body type. They were always seen as "skinny models". Of course there are people with all sorts of preferences, but the most celebrated models of my youth (say, Heidi Klum or Tara Banks) were relatively fit as opposed to withering.

On the other hand, many people (both men and women) end up with eating disorders trying to reach this "ideal".

But we do have to recognize the effects that are also had on people, your last sentence is a heavier weighting than your perception.

People have not been capable of separating an industry ideal from a non-industry related attractiveness ideal.

Individuals know what they like, visually and sexually. Individuals are also aware of what the seeming consensus of "should like" is, visually and sexually. Simple put, people know when other people are considered hot and what that looks like.