The muscle is released from attachments to the rib cage down low and then it elevates away easily to allow the implant's upper edge to be covered. Remembering that the muscle itself ends about halfway down the breast it tends to mostly just provide for compression of the implant up top so the implant isn't so visible and has a better shape in the "upper pole". Size is limited by the need to avoid overfilling the fragile skin that covers the implant. Mastectomy leaves the skin vulnerable to stress in the early period and so a properly sized implant fully fills the breast but doesn't stretch or stress the skin.
Hope this helps,