I am not aware of any studies that were geared at looking at size of the ovaries as they relate to taking birth control
pills, but I recommend that you discuss this further with your OB/GYN. It is true that taking birth control pills suppresses the secretion of gonadotropins such as LH and FSH which are the necessary hormones to induce ovulation. They also induce follicular development which is necessary for ovulation. I suppose that if the follicles were not stimulated for some time, the ovaries would appear smaller. However, this has nothing to do with their function and is therefore an insignificant finding. The fact that you have been told that your ovaries are small is inconsequential as long as they are functioning normally. The only way to determine if they are functioning normally, would be for you to go off birth control and see if you resume regular periods. If you do resume regular periods, then there is no reason to believe that there is anything wrong in the size of your ovaries.
For further questions about this issue, I suggest that you schedule an appointment with your OB/GYN. I think your doctor
will likely give you reassurance rather than be concerned about this finding. Perhaps he or she will want to repeat the pelvic ultrasound