First of all, I think this is the male chauvinism of all men. If a chubby and burly girl stands beside you, it seems that you can't protect her. It's like you want her to protect you. However, if you are a skinny girl who seems to need protection, it will make boys feel a sense of responsibility and mission at once and feel that they are needed. It can also be said to be a feeling of self-realization. I once saw a thin boy standing next to a fat girl in the street. Although boys are much taller than girls, standing together just doesn't feel right. The way two people hold hands will also make people feel that boys will fall down at any time, and girls are his strong support. I think, there should be no boys will like this feeling!
Secondly, I think this should be related to the times. In Yang Guifei's time when fatness was the beauty, everyone probably liked plump girls. But now, it should be said that it is a perverted era in which thinness is the beauty. Many people even sacrifice health to lose weight, but that kind of thinness is unhealthy. Generally speaking, a reasonable diet and reasonable exercise can make them healthy and slim. In fact, sturdy ones are also good, which can protect your feelings for boys and make them healthy and energetic. The best of both worlds is a pleasure!
Finally, I think there is another important reason why boys like thin girls nowadays, that is, they are not strong enough. When I watched some American dramas, I found that foreign girls are generally stronger and stronger than domestic girls. That kind of figure is actually very healthy and energetic, and to be honest, it is also beautiful. However, girls of this figure will look thinner if they stand next to most boys in China, but they still look thinner when they stand next to many foreigners abroad. If a boy is physically active and he is also a bodybuilder, then he doesn't need to worry about what kind of girl will look weak and stand beside him, and everything is so harmonious. For girls, exercising yourself is only good and not bad!