Why some men put so much focus on a woman not being fat?

I have always been okay with woman who was around 30-35% body fat just not someone who looks like they are training for sumo

But i see lot of men online, call even slightly thicker girls "fat".

Does it appear to them, that most women are not supposed to be as Twig like Japanese women/models are portrayed?

In most tribal communities most women don't tend to be so thin.