He is not nice or kind. It is an unpleasant thing to do to try to undermine someone. No one should do it to the person they love.
Normally people that do this are insecure themselves and trying to control. Theory is - She might leave me because I'm shit. If she thinks she's fat, she'll be too paranoid to leave me so she'll cling harder to me. I'll abuse her some more to make her more mine and tighten that noose.
Beyond the fact it is abusive, it's also pointless.
For every woman who is a size 6, there will be a man who thinks she's too fat and should be a size 0. If that's what turns him on, he should date a size 0 woman and leave the size6 woman alone.
For every woman who is a size 18, there will be a man who thinks she's too thin and could do with some more to hold on to. This man should leave the size 18 woman alone, if that's what he wants and date someone heavier.
For every blonde woman, there's a man that wants a brunette.
For every brunette, there's a man that wants a red head.
On and on and on.
People should be not abusive and stay away from people who physically are not their cup of tea.
Dump him forever.