Because, as is clear from observation, he clearly believes that the most devastating way to put a woman down is to attack her physical appearance (i.e. her sexual attractiveness to men). It’s like he thinks all a woman’s worth comes down to one thing, her fucking ability: if she’s not sexy, she’s nothing. A nothing and a person who “perhaps don’t even exist”.
Also, he seems to think that women see themselves that way and therefore will shrivel up and die if they find out that he doesn’t think they’re sexy.
And he is so attached to this notion that feminine value = sexual value, that he applies it to his own daughters. Aloud. No apparent clarifications, warnings or scruples. When he wants to praise The Good Daughter, he says she’s hot. In this case, he wants to insult his daughter, so he says she’s not sexy.
You will also notice that he does not do that to men. Well, in a way, like with women, he attacks men based on hegemonic conformity. Because he’s such a simple man, his standards of conformity are also simple: women aren’t real women unless they’re (their idea of) s-xy, men aren’t real men unless they’re (their idea of) the tomboy. So he will attack men for things like being small, having a soft voice, not “dominating” women, not taking advantage of others, etc. If they are not aggressive manly men (like him, of course), then they are nothing and should be ashamed of their existence.
Fat, however, suits a man. Men are supposed to be visibly successful, take up a lot of space and don’t care too much about how they look! (In fact, his refusal to put effort into appearance shows how bossy they are!) So, in his view, that’s no basis for insult.