Like yeah, your observation is correct, studies have been done on this, but it doesn't mean that the beauty standard is good or natural or correct. If anything it means it's wrong.

Also, today's relative abundance doesn't mean that size is entirely a matter of personal responsibility. Fast food and cheap processed food are often people's only options when they can't afford the time or money to cook meals properly. That food is worse in nutrition so they have to eat more of it, and it's full of junk that capitalists have figured out will be useful in addicting people at the expense of their health, usually a mix of excessive salt and sugar.

There are literal scientific studies about how to make the ideal food that is moreish but doesn't sate hunger, so people will eat lots of it. It takes willpower and resources to fight against that. No wonder there is an obesity epidemic.

So ultimately the beauty standard is still about classism, about wanting someone who is wealthy instead of poor.

If it only exists due to structural inequality where you can have a clear distinction between rich and poor, then it stands to reason that in a world where people generally could expect to thrive and not struggle to make ends meet, so that anxiety about it wasn't commonplace, then a wider array of body types would be generally accepted.

Edit: I hope this comment doesn't read too argumentative. Nothing you said was wrong, I'm just adding to it, and I'm aware that people reading your comment could use it to justify or attack beauty standards, and I'm using it to attack.