This is the "housewives shouldn't have tits bigger than D cups" paradigm that many doctors are leery of (potential malpractice suits), and which in fact most housewives subscribe to. Your girlfriend will need to be very clear about the size issue during her consultation.
As far as the correlation between implant size and cc rating, lots of people make a natural but incorrect assumption that there is a linear relationship i.e. 750cc implants will be 50% bigger than 500cc implants. In terms of cup size/implant measurements, however, the 50% increase in volume actually grows in three dimensions. Mathematically, 750/500 or 1.50 = 1.15 height *1.15 width *1.15 forward projection. Since only the forward projection actually contributes to cup size increase (height and width contribute to fullness and cleavage), 15% more forward projection from 750cc implants versus 500cc implants will actually amount to less than a cup size. Similarly 1000/500 or 2.0 = 1.26 height * 1.26 width * 1.26 forward projection might amount to just about a cup size of increase, but a vast improvement in fullness and cleavage. The fact that a 500cc implant might increase forward projection by 2 1/2 inches, while 1000cc implants will increase forward projection by 3 1/4 inches (2.5" * 1.26) instead of 5 inches one might expect on first impression, has been the source of great disappointment for many girls who have wanted larger breasts but didn't actually wind up with the cup size increase they envisioned. To actually achieve a 5 inch increase in forward projection she would need something like 500 * 1.5 * 1.5 * 2 or 2250cc (given the fact that at some point, implants have to stop getting wider and taller i.e. they adopt a higher profile in order to still fit on the human ribcage).
Also, when going by the "rice test" for sizing, it's important to simulate actual conditions as much as possible. Also, have your girlfriend repeat the "rice" test while wearing a tight workout top to "contain" the implants, the same way her breast tissue will "contain" real implants. This will give her a much more realistic idea of her actual post-surgery size. Girls who merely shove rice bags or "sizer" implants in their bra, without "containing" them, are the ones who are usually the most disappointed in regard to post-surgery actual size, since the natural breast tissue will in fact force the implants to lay flat and there will be no "air space" inside the breasts.