For example, for these boys, they think thinness is a more beautiful state. For example, their legs are thin and slender, and they feel like a weak woman when they wear clothes.
They tend to be physically controlled, and the ideal figure is thin arms and legs. There are also some boys who like thin girls not only because of their figure, but also because of their psychology. As we all know, many boys have male chauvinism. In fact, many boys are born with the desire to protect girls.
If your girlfriend is that strong, it doesn't seem to be to their taste, but a thin girl around will arouse their desire for protection. I want to show my masculinity in front of such a girl.
There are also some people who are influenced by the secondary culture and like thin girls to be otaku. The cartoon characters these people contact are generally of that kind of figure, so in reality they also hope to have a girlfriend like that in the cartoon.