You do not "lose volume" when going under the muscle--you lose apparent size and projection!
As others have stated, your implants have the same total volume whether above or below the muscle. But going below does exert more tissue pressure as well as provide additional tissue "camouflage" of the visible implants.
We know this by having our patients size with implants in a bra and stretchy top. The latter helps simulate the body better than a bra alone, but if a woman selects a certain size after much deliberation and trial and error, and I surgically implant THAT EXACT SIZE, a...