Is there a size difference between # cc in a saline and silicone implants? If so, how much more cc do you need if going to UHP?

Let's start with a quick quiz: Which weighs more-- a ton of lead or a ton of feathers?
Right, you didn't "fall" for that one, so realize that a cc of saline is EXACTLY the same volume as a cc of silicone. Yet, they don't look the same in the human body, even when in identical silicone shells. That is also why they feel different to the touch--most women and surgeons believe silicone implants are softer and more natural than saline, though nice results can be achieved with both!
Your question is a good one and is asked often, so it deserves a detailed answer. 
Once an implant size is selected, then your body (chest and breast base) measurements determine the proper profile choice to get that volume in as wide an implant as possible without folding or creasing. Thus, to get the closest cleavage (and the most upper pole fullness), the widest profile in the chosen size to fit your pocket (with a little wiggle room) will give the best outcome visually. In a 600-650cc implant, for a woman your size, this may be a HP or UHP, but if a HP "fits," choose the HP. The higher the profile, the more round and less "natural" your breasts will look.
Some women prefer the high, round "Nicki Minaj look," and that is the province of a good push-up bra (and having the volume--size--to push up). But nude, a soft teardrop shape is best, and reclining, your breasts should flatten and round out as they drop slightly to the sides of your chest, NOT stick straight up like Tupperware bowls under your skin!
At 5 feet and 93 pounds, these are huge implants for such a petite frame, but you already know and obviously like the look of 600cc implants. Identical volume silicone implants will likely "look" smaller than your 600cc saline implants, and I understand your question is asking for at least the "same" or slightly bigger appearance. The next size up from 600 is usually around 650cc, and your pockets may be at their maximum limit anatomically, so understand that it's not just your (or your surgeon's choice), there are anatomic limits than cannot be breached (though we have all seen examples of women with huge breasts that have been stretched past anything normal or natural). 
If your surgeon can safely use 650cc cohesive silicone implants, that will probably fulfill your goals best! If not, 600cc silicone (HP if possible) will give you the most natural, soft results. For more information, click the web reference below for my Comprehensive Guide to Breast Augmentation. Best wishes! Dr. Tholen

Thank you for the good question.  In my experience, when converting from saline to silicone gel breast implants, a larger volume of silicone gel implant is necessary to achieve the same aesthetic look. I think this is because silicone gel breast implants tend to conform to patient chest walls more so than saline.


 In other words, the volume (cubic centimeters) are equivalent regardless of the material utilized but  utilizing the same volume of saline versus silicone gel breast implants may achieve quite different visual outcomes.   For this reason, I suggest patients undergoing breast augmentation or revisionary breast augmentation surgery (after careful communication of their goals) allow their plastic surgeon ( based on experience  and the use of temporary intraoperative sizers) to guide them when it comes to selection of appropriate breast implant type/size/profile.Remember also that there may be variables (such as the need to adjust surrounding breast implant capsules), besides just breast implant volumes, that may come into play when it comes to achieving a patient's desired outcome.


 I hope this helps.