Are 300cc Silicone Breast Implants Too Big for a Petite, A Cup Woman?

If you are as small as you say, those are big implants. And by my taste,. high profile implants tend to look round and fake, and seem to cause an excessive amount of skin stretch, atrophy of breast tissue, and concavity of the rib cage over time. Of course that doesn't happen to everyone, but one cannot place highly projecting implants into tissue with impunity. There is a price to pay for stretching tissue beyond where it has ever been or is meant to go.
Assuming your base width is 10 cm, then the ideal starting size for you would be about 200cc. If you were loose or empty you would add some volume, and if you were tight or full it would be less than 200cc.
These numbers are based upon peer-reviewed and published data. It is data that has produced the best published results to date in terms of outcomes. If, on the other hand, achieving a certain size is more important to you than respecting your tissues, then you can go larger than that. But understand that there can be adverse consequences to your tissues.

I appreciate your questionAfter performing hundreds of breast augmentations, I have found that the size of implant best for you is dictated by your chest wall measurements.  Once we determine that we can choose the profile based on what you want or need to achieve.  If you are seeking a natural look, then the diameter of the implant should be equal to or, more ideally, smaller than the width of your breast. The breast width is a measurement of how wide your breast is at the base, which should be measured at the level of the nipple. Choosing an implant that is smaller in diameter than your breast width will avoid the "side breast" fullness that is often associated with a more artificial appearance. Implants that are wider than this can be more palpable, fall more laterally overtime and also be more palpable. After that we will then choose the implant volume based on the measurements and also looking at any asymmetries you might have. You should choose a board certified plastic surgeon that you trust to help guide you in this decision. Bringing photos as well can help us to finalize, but please understand that these are of other peoples bodies and your results will probably be different.Silicone will give you a fullness at the top (upper pole fullness). Silicone implants come pre-filled with a silicone gel and are the softest implant available. They feel more natural, which makes them a good option for women with less natural breast tissue; but they require a larger incision. It may be more difficult to realize if this type of implant has ruptured, so it is important to monitor them with annual follow-up visits. Additionally, because this implant contains a more softer silicone (less cross-linked), if this implant should rupture, it may leak but only into the scar capsule formed. However, the newer silicone in these implants may not leak even when the implant shell is ruptured. Anatomic gummy bear implants might be a good choice to give you a more natural look. These highly-sought-after, anatomic implants offer a look that more closely resembles the natural silhouette of a breast, and, therefore, are a very attractive option for individuals seeking a natural-looking, aesthetic primary breast augmentation. Additionally, these implants are an especially excellent option for patients undergoing restorative or corrective breast surgery because they provide more stability, shape, and reduced incidence of capsular contracture. Compared to other types of silicone gel implants, the silicone in the cohesive gel implant is more cross-linked; therefore, should the implant shell “rupture,” it maintains its shape and silicone does not leak. Implants, indifferent projections. The easiest way to think about it is they called me in a low medium and high projection or profile. Most of the time I use the medium profile which gives people the best in terms of natural look as well as a little bit of projection. High profile implants are for someone that wants  A little bit more of a fuller look. Low profile implants are better for someone that really wants a minimal extremely natural augmentation. Depending upon the anatomic versus round implants there are also subtle differences  in terms of the amount of projection for each profile. During your breast augmentation consultation, you should feel the different types of implants available, and try on various implant sizers in front of a mirror to help you to get an idea of how you will look following the surgery. You should also bring pictures of the look you would like to achieve, as well as a favorite top to wear when trying on implant sizers. I also use a 3D imaging system to allow you to try on a variety of shapes and sizes. I prefer this method over adding volume due to implants being under the muscle.We will also discuss the possibility of needing a lift along with an implant. This will depend on the amount of ptosis (nipple position) as it relates to your breast, inframammary crease and chest wall. Needing a lift or not will also determine where to place the scar for the incision.  In some cases if the the breast and nipple is really ptotic (low) it might be best to stage the procedure by performing a lift first and then doing an implant at a second stage. This will also allow you to have a better understanding of the true size and shape of your breast.In order to limit pain from breast augmentation and speed the recovery process I offer patients Botox into their muscles as well as a long acting numbing medication called exparel. The best way to assess and give true advice would be an in-person exam. Please see a board-certified plastic surgeon that specializes in aesthetic and restorative breast surgery.best of luck!Dr SchwartzBoard Certified Plastic Surgeon #RealSelf100Surgeon#RealSelfCORESurgeon

That size does not seem too large based only experience.  If you tried on sizers and they looked good to you, that is the most important factor.