Is it true that silicone implants look smaller than saline? Replacing 275cc Mentor mod saline and want to be same size

Very good question. Saline implants are filled at the time of the surgery with certain volume for example 275 cc in you your case, and this is recorded as the implant volume. This volume doesn’t include the volume of the implant shell that could be 30 or 40 cc. The only saline implant company that gives you the shell volume so that you can record it as the total implant volume is the Ideal Implant Inc. If you have a saline implant and want to go with a silicone or Ideal Implant, I would just pick an implant that is 30 cc larger than the one you have in order to keep your volume the same. The choice of profile will depend on your present implant and your dimensions and it can be decided during your consultation. Just make sure you bring all the pertinent information and any before and after pictures.

Thank you for the question.  Although precise advice would require in-person evaluation, generally speaking, I do find that it takes different volumes using saline versus silicone gel breast implants to achieve the same “look”. Usually, a larger volume silicone gel breast implant is necessary to achieve the same look that is achieved with a saline breast implant (at least in my experience).
Also, 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. 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. 
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. 
I hope this helps. 

There are so many factors which can  influence the shape of your breast following augmentation in addition to  implant  volume. If the implant is placed in a sub muscular position and the pocket has been made precisely, I believe the silicone conform to the space much better than saline. The  difference of 30cc can not be detected between the two implants.  See an experienced PS .