First, "flat chest" will look more advanced and look better to wear. In fact, supermodels not only have "flat breasts", but also "big breasts". Victoria's secret girls are basically "flat-chested". The main reason is that "flat-chested" is more advanced. They think that this can better show their brands, bring good feelings to their underwear and show the most beautiful side of their brands.
Second, there are many styles that "flat chest" can control. Generally, girls with big breasts have certain limitations in wearing clothes. Because their breasts are too big, they all have certain limitations when choosing clothes. The style is relatively simple, but the "flat chest" is different. They can control more styles, such as small fresh style, domineering style or other styles, and they can control them very well. They are not so limited and have a wide range of development routes.
Third, because the figure is relatively tall and thin. I believe everyone knows the phrase "the weight does not exceed 100, and it is either flat-chested or short". Similarly, most of the girls in Victoria's Secret are taller and thinner, so it may be their physical characteristics that cause their breasts to be not too big and become "flat chests".