At your height and weight, and depending on your breast examination, 615cc implants may be just what you want, or possibly on the somewhat too big side of things. If you truly "fill out" a C-cup bra now, you will be a DD or larger cup size. If this is what you want and your anatomy allows your chosen implant size, then I'm all for patient satisfaction. If you wear, but do not adequately fill a C-cup bra, you may well end up with a D-cup to DD in size. But it's not about letters on a bra label, it's about getting the "look" you seek!
I do not believe "big implants bottom out," or "big implants have more problems." Rather, if they fit your anatomy and goals, then it is my job to place them and securely close your tissue layers to minimize the risks for any kind of problem leading to dissatisfaction. I'm not sure why some of my colleagues feel it is less of a problem to have an unhappy patient who is too small (enough that many have come to me over the years for different implants via another operation), than one who has the breast volume they seek and a potential for other issues that skillful surgical technique can usually avoid!
If you are sure what you want, or are getting confusing or conflicting information, seek additional consultations with several other ABPS-certified plastic surgeons and discuss your goals and concerns; after which it is up to you to listen to your consultants' recommendations. You can then best choose your surgeon (or perhaps choose your best surgeon), who should earn your trust, and then do everything surgically possible to come as close as feasible to reaching your goals.
BTW, in appropriate patients who choose large(r) implants and have the anatomy to allow it, I utilize 700-800cc implants. They DO look big in many cases, but large, soft, properly positioned, and non-capsular-contracted breasts can look very nice and make that patient quite happy. Best wishes! Dr. Tholen