Question about implant size: 375cc vs 400 cc? Are the sizes seen with sizers accurate? (Photos)

Choosing an implant size/style is an important decision which is best made in concert with your surgeon. Ultimately, options are based on a number of factors including:
-Chest dimensions: This point is crucial as the width of your breasts defines the available "real estate" on which the foundation of your augmentation is built. This "foot print" is the absolute limit with which you are dealing. Too "large" is not so much the risk as too "wide." You cannot build a 3 acre mansion on a half acre plot. An implant which is too wide (exceeding the available space on your chest) will generally over hang laterally making it more noticeable and occasionally contributing to discomfort and nerve symptoms (numbness or tingling). This will also contribute to a more unnatural look.
-Tissue quality: This relates to size in the sense that the implant will be suspended by your tissue. Too large an implant may place took large a stress on these suspending tissues leading to healing problems in the short term and bottoming out in the long term.
-Patient desire (what is the look you are hoping to achieve?)
My role as a surgeon is to manage expectations and guide you towards a prudent choice which can satisfy these reasonable goals. Often times, a surgeon can guide a patient toward a size/cc range and patients will make determinations on implant style/profile. Patient seeking more or less projection, or seeking a more pronounced or subtle upper pole volume increase can be guided toward an implant which will help them best achieve their goals. Additionally, technique variations (sub muscular vs. sub glandular) can also aid in obtaining the look you desire.
There are a number of tools available to help guide patient/surgeon decision making including sizers, 3D imaging and goal photos. Each modality can help better communicate your goals to your surgeon and vice versa.
Ultimately, it is important to not marry oneself to a particular "cc" as oppose to a particular "look." Communication is key as you and your surgeon create your custom surgical plan.
With regards to your specific question, given your measurements, your implant choices sound large. None of the available sizing options are 100% accurate. But they are useful tools. Volume is not necessarily different post op but shape and projection will change (leading to the tendency to go bigger).
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Unfortunately, you have not provided photographs or told us your bra size. At size 32 or 34, each 100 cc of implant corresponds to 1 cup size change. At size 36 or 38, each 200 cc of implant corresponds to 1 cup size change. From this, you can compute volume required to achieve your desired goal. 400 cc implants weigh 400 gram, will result in descent and will be very heavy. I recommend smaller implants to be more stable long term.

Best Wishes,

Gary Horndeski, M.D.