Breast size is a common concern for many women, and I am sorry this is an issue for you. Breast size has many determinants, including genetic factors, overall body mass, and hormones. I encourage you to consult your physician to help determine if there are any specific medical conditions that may be affecting your breast size.
In general, breast size increases during puberty due to increased production of estrogen. The onset of puberty varies widely, and some young women achieve the maximal breast size at a younger age than others. Body mass can also affect breast size. Body fat is often deposited in breasts and can ultimately lead to larger breasts. Women that exercise a great deal can cause delayed puberty and smaller breast mass.
If you are having difficulty gaining weight, this could also be due to an abnormality of your endocrine system, such as thyroid dysfunction. In addition, certain genetic abnormalities can cause abnormal estrogen production, which in turn affects onset of puberty and breast mass. It is important for you to undergo a thorough history and physical exam by your physician. Depending on this evaluation, you may need to undergo additional blood testing. Assuming there is no specific medical condition causing breast size smaller than you would like, there are no safe pharmacologic options available. Surgical breast augmentation is a possibility, but you would need to consult a plastic surgeon
to determine if this is an option. In general, at your age, it is very possible that your breasts will continue to grow.