Many girls don’t like their breasts. They want them bigger or smaller or more firm.
The size and shape are determined by your genes. This is outside your influence.
The idea that you can change the shape or size by doing exercises is a fiction. The breasts contain no muscles, except for a bit of muscle around the areola. There is only one big muscle, but this is located outside the breast. You can exercise this muscle behind the breasts, but this will only increase the size of your chest, as with bodybuilders.
Hormones or other medications that are advertised, for instance on internet, will have no effect on the size of your breasts.
Breasts can only be enlarged or reduced in size by plastic surgery.