You are petite and slim, with A-cup breasts. Every surgeon measures breast base differently, which is understandable since breasts do not have a "sharp" beginning and ending, but slope as they attach to the chest wall. Thus, this is not an "absolute" measurement, and implant profile choice should be made on the basis of the pocket created, which of course is based upon the breast beneath which it lies. So there has to be some degree of consistency, but each surgeon makes this judgement on the basis of his or her own measurement "method," which works for that individual surgeon's decision-making process.
I suspect that if I were to measure your breast base diameter, I would measure this slightly larger than 11.0cm, and this would perhaps allow a moderate profile 300cc implant, or at least a moderate plus profile implant. A Mentor 300cc moderate profile saline implant has a base diameter of 12.6cm (overfill to 325cc, diameter 12.1cm); a moderate plus (300cc) 11.5cm (overfill to 360cc, 11.2cm), and a high profile (310cc) 110.cm (overfill to 375cc, 10.7cm). Allergan Natrelle 300-320cc low profile saline implants have a base diameter of 12.4cm; moderate profile (300-330cc) 11.9cm; and high profile (320-340cc) 11.1cm. Remember that saline implants must be over filled by 5-15% in order to minimize rippling, which IS an issue with someone as tiny as you are, and why both companies have a range of volumes listed with corresponding (slight) differences in base diameter.
So you can see that these profiles differ in base diameter by less than 1cm, and even differ between manufacturers by only a few millimeters. Just as measurements might differ among surgeons.
I think that some surgeons prefer the "high, round, Baywatch" look of high profile implants, and tend to recommend them to most if not all of their patients. Saline implants tend to have even more "sharp" edges and look less natural than silicone, especially when above the muscle. Yours should absolutely be below the muscle, IMHO!
Overall, 300cc (even overfilled to about 330cc) will add about 1 1/3 cups to your present size, so you must be as sure as you can be that this is the proper implant choice to give you the full C you wish (I suspect you may be a bit on the low side).
Secondly, I would recommend silicone over saline, especially as you are tiny, thin, and with little of your own breast tissue to conceal implant edges and rippling. You need more information and perhaps another few consultations with ABPS-certified plastic surgeons before proceeding. You can get much more information by reading my Comprehensive Guide to Breast Augmentation by clicking on the web reference link below, or by coming in for a complimentary consultation. Best wishes! Dr. Tholen