Not happy, with 245cc motiva demi - should I replace with 320 or 340?

In my opinion, both 320 cc and 340 cc Motiva demi implants will give you more volume than 245 cc, but 320 cc is more likely to keep you in a natural range that fits your goal of wearing bralettes (small) and keeping your xs/s shirt size. The difference between 320 cc and 340 cc is relatively small, but it can be noticeable on a petite frame.

Since your priority is fuller without looking “too large”, 320 cc tends to be more predictable for that sweet spot between enhanced and natural. A careful in-person assessment of your chest width, tissue characteristics, and pocket will help confirm which size best matches your goals.

Whenever you’re ready:
Let’s make it better.

Warmly,
Dr. Luis Mejía, MD
Board-Certified Plastic Surgeon
CECILIP – Dominican Republic

Choosing an implant size/style is an important decision which is best made in concert with your surgeon. Ultimately, options are based on a number of factors including:
-Chest dimensions: This point is crucial as the width of your breasts defines the available "real estate" on which the foundation of your augmentation is built. This "foot print" is the absolute limit with which you are dealing. Too "large" is not so much the risk as too "wide." You cannot build a 3 acre mansion on a half acre plot. An implant which is too wide (exceeding the available space on your chest) will generally over hang laterally making it more noticeable and occasionally contributing to discomfort and nerve symptoms (numbness or tingling). This will also contribute to a more unnatural look.
-Tissue quality: This relates to size in the sense that the implant will be suspended by your tissue. Too large an implant may place took large a stress on these suspending tissues leading to healing problems in the short term and bottoming out in the long term.
-Patient desire (what is the look you are hoping to achieve?)
My role as a surgeon is to manage expectations and guide you towards a prudent choice which can satisfy these reasonable goals. Often times, a surgeon can guide a patient toward a size/cc range and patients will make determinations on implant style/profile. Patient seeking more or less projection, or seeking a more pronounced or subtle upper pole volume increase can be guided toward an implant which will help them best achieve their goals. Additionally, technique variations (sub muscular vs. sub glandular) can also aid in obtaining the look you desire.
There are a number of tools available to help guide patient/surgeon decision making including sizers, 3D imaging and goal photos. Each modality can help better communicate your goals to your surgeon and vice versa.
With regards to your specific question, I recommend a 100 cc change if you are seeking a visible upsize. It is important to realize that small differentials are unlikely to make or break a result. A shot glass is slightly less than 50 ml. 320 vs. 340 are likely to be indistinguishable.
Ultimately, it is important to not marry oneself to a particular "cc" as oppose to a particular "look." Communication is key as you and your surgeon create your custom surgical plan.
As always, discuss your concerns with a board certified plastic surgeon.