Size correction with only 10cc more?! What?!

Hi MommaAA.  I agree-10 cc is a small amount, and would not compensate for significant differences between breasts. There are differences in the musculoskeletal frame, breast tissue and skin envelope from side to side in every woman.  The implant will not be able to compensate for everything, so persisting asymmetry is normal & expected. The diameter and projection differences in the implants of 345cc & 335cc, if any, are on the scale of 1-2 mm.  Keep in mind that saline implants have a range in which the volume of saline can be filled, per diameter & projection-perhaps that led to the recommendation?  With silicone gel implants, you have to choose different implants altogether,.  Consider trying on the two sizers or imaging software to estimate the outcome.  Lastly, it is not common to charge a separate fee for "correcting" differences between breasts-it is a standard approach to optimizing outcomes. Talk to your PS about your concerns.

Thank you for the question.  Although much depends on the specific patient's body type and current breast asymmetry,  a 10 cc difference in breast implant volume rarely creates a significant amount of a difference to be clinically noticeable.
***Given that your surgery is coming up soon, I would suggest that you spend additional time communicating your goals/concerns directly with your plastic surgeon, preferably prior to the date of surgery.
 Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. ***Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. For example, 
I have found that the use of words such as “natural” or "C or D cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
I hope this (and the attached link, dedicated to breast augmentation surgery concerns) helps. Best wishes for an outcome that you will be very pleased with.