Anticipated Size Not Achieved After Breast Augmentation

The bottom line is that you need to be happy with your breasts after the expense and discomfort of breast augmentation surgery, and you should not have to go to the trouble of push up bras to look the way you want to look.
It is true that you will look larger and fuller after 3 months when the implants drop and more fully distend the breast. So wait 3 months before deciding.
Generally, people who have "A cup" breasts and want a full C cup breast usually require at least a 350-375 cc implant and at least 400-450 cc is required to get to a D cup.
Express your concerns to your surgeon, but give him/her the benefit of the doubt and wait 3 months.
Did you express any concern about 'not wanting to be too large" to your surgeon before surgery? Most plastic surgeons tend to err on the large size. in my experience patients who end up feeling too small after breast augmentation have tended to influence me to go too small because they were fearful of being too large.
If this is the case then you share responsibilty for the result. I am sure your doctor wants you to be happy, and will work with you to place larger implants if you are not happy at 3 months. However, the expense of implants, OR, and anesthesia are considerable and hopefully you can work out an equitable financial arrangement with your doctor if replacement becomes necessary.

It's not uncommon for patients to express concern about breast size in the immediate post-operative period. Multiple factors make it impossible to assess breast size one week after surgery.Several factors affect the size of the breasts in the immediate post-operative period. These include swelling of the breasts and surrounding tissue, muscle spasm and the healing process itself. These factors interact in a complex manner to create a rapidly evolving situation. It's virtually impossible to make decisions about breast size until the situation has stabilized.In some cases, it may not have been possible to place a larger implant at the time of the original procedure. During surgical procedures, it's not uncommon for surgeons to make compromises to obtain the best possible aesthetic result. In some cases, utilizing larger breast implants can cause conflicts with other aesthetic goals. For instance, having large implants might create an unnatural look, which may not be desired by the patient. It may disrupt harmony, balance and proportion with other surrounding structures.All of these concerns might place limits on implant size. In most cases, after a period of adjustment patients are happy with their breast size. Rarely, secondary procedures are necessary. For these reasons, it's important to maintain close contact with your plastic surgeon.

In my experience, if a woman wears a size 36 AB bra and would like
to be a C cup, which is the average size that women wear and the average size
bra that is sold, she needs a volume of approximately 350 to 390 cc to make
that change. This becomes less if she's wearing a 34 bra, because it has a
smaller strap size and the cups have a smaller diameter. A smaller diameter
implant would then be a volume of approximately 300 to 350 cc. In the same way
if she was more petite, maybe a double zero or size one and wore a 32 AB bra,
then to become a 32C cup she would only require about 250 to 300 cc.