For a petite person, 375 cc implants are rather large. It's also rather imprecise to discuss cup sizes considering that this system of measurement is highly inaccurate due to manufacturer differences and improper bra fitting. Because of this, most surgeons rely on chest wall measurements to select implant sizes.
Along with assessing breast skin tightness, surgeons can better determine appropriate implants for a given person's anatomy. Chest wall diameter should match implant width or the implant will be too wide and encroach upon the armpit area.
Higher profile implants allow for narrower diameters and more volume, and I'm sure your surgeon plans on using a higher profile device. Other than width, volume should match what the soft tissues can safely handle. Previous pregnancies and looser breast skin allow for some volume increase, but only a careful breast exam would provide these details. Despite providing large volumes and narrow diameters, some high profile implants have too much projection and look unnatural on given patients.
Finally, implanting inappropriately large devices causes irreversible changes to the soft tissues due to constant pressure from the implants. This may be an acceptable surgical plan, but without the other details, this seems like an aggressive augmentation that may have consequences down the road. If every 100-150 cc's equates to added cup sizes, a small D result seems underestimated in your case.