'Fat' should not be used in kids books as a derogatory, or at all frankly. Fat is an incredibly important and necessary part of a healthy diet and should be treared accordingly. Training people to think of fat in the antiquated notions of bad cardiogy isnt useful.
So while this may have been a move centered around "body positivity", it serves and higher purpose.
Food fats aren't bad. So call them chubby or overweight or whatever you want. Using the word fat, when fat refers to a food product, gives dumb people the impression that fat is a bad thing.
Do you get this concept?
"Thin" isnt a food group. Short and tall, also not food groups.