The societal default is white hetero male; it has a gravity well, and writing anything else is pushing uphill. We live in a kyriarchal society: it informs and infects all of us, whether we "honestly" believe in diversity or not.
(I don't have to believe in diversity. It's demonstrably real. It exists. It's a thing. I believe in representation.)
The thing you are accusing me of in your last paragraph is exactly the mindset I am arguing against: that there is something *wrong* with not being "the default," and that the story has to be about that character not being the default to have any validity.
Body type acceptance is indeed a major issue in fiction, and fat=evil is one of my least favorite tropes.