Fat woman offended because book character isnt fat enough. People scream slay queen

I just finished the book Jemima J, and the main character is a plus-size woman. She has a quadruple chin, people stare at her on the street, and she needs to catch her breath after walking up the stairs. Then toward the middle of the book it's revealed that she is 5'7" and her highest weight is 217. I'm not saying a person of that size wouldn't have any issues, but it seems like the issues described would be unlikely.

Similarly, in the book She's Come Undone the main character is 5'6" and weighs 257. She needs a special chair in class, she is too big even for plus-size stores and when she gets in a car it tilts because of her weight. These experiences also don't seem to be accurate for someone of this weight (the book is set in the 1960s/1970s during these things, so I understand views on weight and average sizes were different. But still).

I can think of a lot of other examples as well where a character seems to be having the experience of someone 100 or more pounds heavier.

Any ideas as to why authors often get this so wrong?