Finding the right size
It is very difficult to determine the exact size and shape implant you will require to best match your ideal breast image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your breast will determine, in many cases, the maximal volume per implant profile that you can accommodate. To illustrate; a 100 cc difference may make a significant difference with a narrow base width breast, but much less of a difference if you have a wide chest wall and wide breast “foot print”. Therefore, just because your friend may have a great result with let’s say a 350 cc implant to make her go from a “A” cup to a “C” cup size does not mean that you will have the same result with the same size implant. . Further simply placing implants in a bra to determine the size best for you is not always accurate as the bra often distorts the size, is dependent on the pressure the bra places plus the implant is outside your breast and not under it among other variables. Computer software morphing programs that automatically determine the best implant size can be helpful in some but not all cases (e.g. doesn’t work well in my experience with existing implants, sagging or asymmetric breasts). Using “want to be” photos however are useful if simply provided to the surgeon as I will further explain in the below link.
You can always have more volume
but would that really help with improving your appearances? You are on the shorter side and already had 'heavy looking' breasts. Bigger is not always better under these circumstances. If you really wanted a fake look, you would have to consider having a reduction at the same time as an augmentation and it has its pros and cons. This would minimize your risks for your own tissue to hang over the implant as it is trying to do.
Unhappy Augmentation Mastopexy
Your breasts were already large and you added a relatively large saline implant, which acts like a 'water hammer' on your tissues. The implants are dropping, and so is your breast tissue. To achieve the look you probably wanted (orb like with full upper poles) would have required a substantial reduction of your natural breast tissue in conjunction with a larger implant, preferably silicone gel.
You will need a revision surgery to substantially change your breast appearance. Doing another skin tightening procedure without a reduction or adding more saline to your implants will not help.
Best of luck!
Loose breast skin
In my experience, a large breast with a large breast implant and a mastopexy always equals an early recurrence of the breast laxity. It makes sense that if your breasts didn't hold up when they were the size that they were that they probably won't when you add more size. I like to remove lots of skin, some of the redundant breast tissue and place a rather modest size implant which won't aggravate the gravitational descent of the breast. It appears from your side view, but the breast implant has remained high up in the sub muscular pocket but the breast gland has fallen off of it. You may need to have a touchup of the mastopexy. Good luck.
Adding more fluid to# breast implants
Technically the answer is yes, however that voids the implants warranty ( if company finds out) also the amount that you can add to the maximum fill of the implant used, may not give you the increase in volume you want.
This is way in my practice the patient makes the decision about the size of the implants, using sizers, it sounds like your surgeon and you decided to go with a implant, smaller to what you really wanted, and now the only way to make your breast larger will be with a revision surgery, for which you need to wait at least 6 months.
The most important factor in obtaining a good result in your cosmetic surgery is the surgeon you elect to perform it.
Thank you for your pictures. You're correct, your breast are somewhat droopy. Unfortunately, when women have a lot of breast tissue and desire a lift and a rather large implant like you have there's going to be some group overtime. This is due to the nature of your skin and weight of the implant. The only way to achieve good shape and no droop in my opinion, would be to significantly reduce your breast with a modest size implant. Good luck