Because of the aesthetic influence of the entertainment circle, girls want to have a slim waist, so as to achieve the realm of "dressing thin and undressing fleshy". It will make them feel more attractive, and the muscles all over will make girls look "strong" and lack the femininity of women. Most girls don't want to grow up like that.
Boys, on the other hand, pursue masculine beauty, and more abdominal muscles make them look stronger. To satisfy the heroic dream of childhood, proper muscles can also make girls feel safe.
Of course, there are also some girls who like to gain muscle more than boys. These girls think that proper muscles will make their bodies stronger and look healthier. Of course, I don't want to practice the image of a weightlifter here, but the slender long legs and softer muscle curves on my arms will look better.
Boys don't like to gain muscles, not because they don't want to have muscles at all, but like those girls who like to gain muscles, they hope to have some muscles properly without being too "fierce", because the massive muscles like weightlifters don't look so cool and handsome.
In fact, most people who exercise are like this. They want to have a good figure, and proper muscles can make them realize their dreams, which really makes them "muscle men". Forget it.