Do the implants lose 50cc when placed under the muscles?

Thank you for the question. No, in my opinion, it is not possible to accurately determine how many ccs (if any) a patient would “lose” when undergoing sub muscular breast augmentation surgery. I suggest patients do not make decisions based on statement such as: “you will lose 25 or 50 cc of volume when breast implants are placed under the muscle”. These statements are simply not reliable enough to utilize clinically. 
 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. Sometimes, it is necessary to seek several consultations before you feel comfortable about your choice.
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. Again, best not to discuss your goals and/or judge the outcome of the procedure performed based on achieving a specific cup size. The use of computer imaging may be very helpful during the communication process. The use of in bra sizers may also be helpful. In other words, use as many “visual aids” as possible during the communication process. I encourage patients to meet with me as my times as necessary, to feel comfortable that we are both on the “same page”.
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 generally select appropriate breast implant size/profile after the use of temporary intraoperative sizers and viewing the patient's chest in the upright and supine positions.
I hope this helps. Best wishes.

There are number of ways surgeons will try to convey a sense of post augmentation size in the pre-operative period. These include 3D imaging and sizers. While helpful, they are not perfect.
The use of sizers can help give a sense of volume increase when placed in a bra. However, placement underneath tissue will alter the shape. While there is no exact way in which to quantify this shape change, patients should be aware that there is a change in projection when placing an implant underneath the muscle which is difficult to predict. I have heard a number of theories ranging from a % drop in projection to a purported volume drop. These are not scientific correlates with no consistency. I have not found these to be helpful.
Patients should be aware that how they appear in a garment is not exactly how they will appear post operatively. 3D imaging is helpful in accounting for this variation. Ultimately, at the end of the day, this remains a computer simulation.
Ultimately, even if submuscular placement did account for a perceived volume loss, 25-50 cc is a subtle change and equates to the volume of a shot glass (at most). 
Implant selection is a process which should involve both the physician and the patient, using good communication to take full advantage of patient goals and surgeon's experience.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).