The thing is that if someone is being shamed, stigmatized, bullied etc. for being fat, and we say “they aren’t fat” or “they aren’t even that fat” in their defense, what we are also saying is that there is a size at which they would deserve that treatment, and that’s just not true.

Countering fat shaming by denying fatness says that the person doesn’t deserve poor treatment (which is true) but at the expense of reinforcing the incorrect idea that they would deserve it if they were fat (or some greater degree of fat), or that being called fat is an insult. There is no size at which people deserve to be treated poorly.