Breast size varies among females and size may be due to family history and is also deoendant on hormones levels.
It is also dependant on physique of girls i.e. lean and thin girls have small breast size.
If so than I will advise you to increasr your wait by taking high energy foods and increasing the amount of food intanke. I Sexual feelings also help in increasing the breast size through changes in hormone levels as size of breast increases in many females after marriage.
over the bteast stimulates the breast and helps in increasing the size.
You should also start some exercises which increase the muscle mass of chest so making the look of your breast more prominent.