For example, when I was in junior high school, my height was 167cm, and my weight remained around 55kg all the year round until I graduated from high school. I didn't deliberately diet or exercise. I remember that if a girl had 60kg at that time, she would be regarded as fat by her classmates. Now I am 26 years old, and I feel that I am obviously fatter than my girlhood. Plus exercise and diet control, I'm still ten pounds heavier than I was seven or eight years ago. Those square dance aunts in their forties and fifties exercise much more than young people, but their weight is per capita 130 kg. Women with better conditions, even in middle age, will try to stay slim, the biggest purpose is to look young.
A similar aesthetic is that women wear wrinkle needles, risorius, Lolita skirts and jk uniforms. The purpose is to shape himself into a lovely "false mother". Of course, to look like a young girl, you must be slim.
As for why young women are beautiful, it may be related to reproductive value. It will be more difficult for women to have children after 35. When a man marries a beautiful girl of 20 years old, it means that he may have more offspring than those men of the same age who marry a woman of 30 years old.
Since human beings have classes, a well-off spouse will bring more benefits to himself than a spouse who is good at manual labor. Therefore, many times, popular aesthetics will be "aristocratic" rather than grounded. Similar aesthetics and female foot binding mean that the family is superior and there is no need to work in the fields. Women's white skin means that their family is superior and they don't need to work in the fields. A woman's oval face means that she eats more delicate food and doesn't need to chew. Of course, a thin woman means a better family and does not need manual labor. This is also the reason why white and thin beauty attracts men more than strong and black peasant women.