How big is 700cc and is that bigger than a C cup after mastectomy?

Trust your plastic surgeon. No implant based breast reconstruction can achieve a DDD size. Your reduction toward a "C" may end up a "C", or a "D". Cup size is variable and inexact. Currently the largest implants are around 800 cc's and your final cup size with 700 or 800 cc's is unlikely to be too large if that is what your plastic surgeon recommends. Trust him or her, and it's best to discuss your concerns in that setting. Best of luck to you.

Each patient is different, but a 700 cc implant is pretty large and very likely going to put you into a cup larger than a "C" although implant volumes do not match up to cup size.