They think that way about many things including being overweight. I don't mean medical theories but attitudes offered by society and the media. You should be super-model slim. You're not a great person if you are overweight.
Slim people are better people.
Being overweight isn't unhealthy. Don't be concerned about it. We make too much about being slim, anyway. I hope you can see that both of these theories are erroneous. He or she should lose weight for health reasons.