Booked to have 300cc Demi motiva ergonomix implants. Can anyone advise on sizing?

I am booked in to have 300cc demi motiva ergonomix implants under the muscle in October but have started to worry they won’t be big enough. I am about 5’7” and weigh 55kg with very little breast tissue, I don’t even fill an A cup. I would like to achieve a full C or a small D cup. Looking at photos I like 360/380cc size. My surgeon seems cautious about going too large. Can anyone advise on sizing?