Your breasts are bigger, which means they are well developed. It may be because the chest is relatively large, so it is prone to external expansion and sagging. Now that you are young, you can adjust. It is suggested that you wear better quality adjustable underwear, and the method of wearing underwear must be correct. I suggest you try more underwear stores and consult the clerk. If you don't know the correct way to wear underwear, the fitting room of underwear store usually has a method map. Please have a look.

Hope to adopt!