"Fat" is an insult. It isnt hard to understand. Is it factually true that an obese person is 'fat', sure. It is still generally regarded as an insult.
Most of these replies are coming from idiots. This is both factual and an insult.
I havent made any remarks defending obesity or suggesting we need to curtail efforts of obese people to control their weight
Calling a character in a kids book "fat" doesnt help kids stay healthy and fit. Insulting and shaming kids doesnt help them stay healthy and fit. Their parents probably feed them garbage and have made poor lifestyle decisions.
That a good diet includes some fat is orthogonal.