Thin privilege is reading (or writing) a book and having your body type be the default image in your head for the protagonist.
It is really hard knowing that my body type is only used for villains or…
The issue isn’t simply the reader ‘imagining’ a character the way they’d like the character to look, the issue is that novels rarely feature fat protagonists who are specifically described as such.
That’s like telling someone that you can simply ‘imagine’ that the lead character in the book is someone of color just because their race hasn’t been specifically stated as white. Yes, they technically can ‘imagine’ the protagonist as black,native American, Chinese, middle eastern or any race but that doesn’t negate the fact that people of color rarely are featured as a protagonist.
Just because you can ‘imagine’ the world you’re reading any way you want to, doesn’t mean that it suddenly negates the demographics that are pushed to the back burner in favor of the ‘conventionally attractive’ protagonist image.