Under VS over the muscle. What's the difference? Undecided if I should 600cc or 650cc?

50cc is less than two ounces, I do not think you will notice a significant difference in those two implants.  Many people will advise you to go larger because patients will often say they wish they had gone larger. In my opinion going smaller makes sense, especially with very large implants.  Less weight on the tissue gives you a better long term result.  Also, under the muscle provides a more natural result with  a better long term result, in my opinion.  Good luck, Jane. 

Thank you for your question.  Given your height and weight, I think a 600cc or 650cc implant will be very large for you.  Regarding the placement of the implant above or below the muscle, I think either position can give a very natural look.  It really depends on how much native breast tissue you have, to conceal the implant.  There have been some well done studies demonstrating a slightly lower risk of capsular contraction or with the implant being placed below the muscle, however the downside to beneath the muscle is the risk of animation deformity and lateral displacement of the implant overtime.  Good communication, with your board-certified plastic surgeon where your expectations are clearly conveyed through the use of pictures, and an opportunity to try on different breast implant sizer's can be very helpful.  Your plastic surgeon will take measurements of your breast, determine how much of your own breast tissue you have, and discuss the most appropriate position for the implant, weighing the pros and cons of each position with you.  The 'look' you are hoping to achieve (for  example, more upper pole fullness) should be carefully discussed with your plastic surgeon.  Ultimately, when you are down to one or two sizes as it seems you are, give your plastic surgeon the freedom to place different sizers within the breast pocket created at the time of surgery and determine which size will look best for you.  It really comes down to excellent communication between you and your plastic surgeon.  Clearly convey your expectations and concerns  and don't be afraid to seek a second or third opinion.  Good luck!