Is there a size limit to under the muscle implants? 34AA, Athletic, 5'8', 70kg, no kids (Photos)

Hello and thank you for your question. You are a great candidate
for a breast augmentation.  The size, profile, and shape of the
implant is based on your desired breast size/shape, your chest wall
measurements, and soft tissue quality.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.  This entire surgery
can be performed with a small incision technique.  Make sure you
specifically look at before and after pictures of real patients who have
had this surgery performed by your surgeon and evaluate their results.  
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon

Based on your pictures, it does appear that your own tissues will be the limiting factor in the size of the implant.  A HP implant will provide the projection for improving the shape, but the size is really based more on measurements.  I would caution on the use of saline implants though.  When you are thin and have little breast tissue of your own, there is a higher chance of rippling issues with a saline device.  I hope this helps.