One of the most important reasons is definitely aesthetics. Women please themselves, and women are good-looking for boys after all. Many foreign men like girls with big hips and particularly explosive bodies. They think that is the most beautiful and attractive. However, most men in our country can't appreciate girls like that. Although our ancestors left the motto of "big ass", we still prefer girls who are weak and slender.
Historical reasons. Our country has a long history. Except for a few dynasties such as the Tang Dynasty, fat is beautiful, but it just likes fullness. China often mainly appreciates women's femininity, emphasizing that women should be virtuous, gentle and considerate, and be mainly responsible for housework. And what clothes in China need a slim figure to wear the essence. This is a question of historical inheritance.
Perhaps their concept of equality between men and women appeared before us. Women don't have to rely on men, so women also do fitness and strength training. This is an obvious performance. Although our ancestors' concept of gender inequality has been abandoned, some derived concepts still cannot be changed quickly. Therefore, although some women in China have started to exercise, they are still in the minority, and most of them still value thinness.