I would like to read the arguments against this point of view:

Fat women are not direct competition for slim ones, nor for slightly overweight women in the dating market and in terms of male interest. So why shouldn't thin women encourage fat women to be happy with their fat, eat more and stay fat?

Moreover, if most women are fat, then even normal men will have to be with fat women. So then it is enough for a woman not to be fat and not to be downright ugly, as a ticket to desirable men.

It is therefore understandable that the body positivity movement has taken hold among women. But it's not proof that women like each other or support each other.