Change in size of one breast than other breast is normal specially in teenage . It's not a disease and doesn't need treatment. Since you have already done all necessary investigation and had a physical examination done by doctor you don’t any other investigation at present . If there is any other symptoms other than the difference in size or any other difficulties, please fell free to discuss it over consult . No need to get stressed as these are normal physiological changes . If you have family history of any particular disease or any other known risk factor please let me me know , can give advise regarding how to plan for screening in future . Other than that no need to do anything now .