Choosing the right sized implants: how to choose the right size for patients who already have a lot of breast tissue/breast fat?

Search for board certified plastic surgeons who perform hundreds of aesthetic breast procedures each year with favorable patient feedback and hundreds of before and after photos on their websites.  Select a couple and schedule consultations with them.  They will be able to guide you to the correct size for your measurements and desires.  Measurements of the chest wall and breast imprint width as well as knowledge of all breast implant sizes and measurements should make this process easy.  The difference between these sizes is difficult to appreciate, and you should pick the one you like better.  
Try on a profile bra with the sample silicone and saline implants under a tailored blouse, tight sweater, swimsuit, and other garments you typically wear so that you have the best idea of whether the garments fit as desired with the implant selected.  I would recommend that you go with the size that boosts your self-esteem, and fits your anatomy.
Kenneth Hughes, MD, Board Certified Plastic Surgeon
Los Angeles, CA

Thank you for the question.   
Ultimately, careful selection of plastic surgeon and careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging)  as well as careful measurements (dimensional planning) will be critical.
 ***Given your concerns, I would suggest that you spend additional time communicating your goals/concerns directly with your plastic surgeon, preferably prior to the date of surgery.  It will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.
Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. For example, I have found that the use of words such as “nice shape and cleavage” or "DD cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
I hope this helps. Best wishes.