Thank you for your question, and I am sorry that your breast size is causing you concern. Much like height or hair color, there is a wide spectrum of breast size and shape in the human population that is largely determined by our genetic material. If you are interested in pursuing a larger breast size a breast augmentation with the use of implants may be a possible option. See a board certified plastic surgeon who can discuss with you the pros and cons of such a procedure.
Some women are just born with smaller breasts. A breast augmentation should help. Please seek a board certified plastic surgeon for a consultation.