User blog comment:B214/Talk About What You Want! 3/@comment-101.175.92.231-20150218091212/@comment-16281343-20150219061924

Evil Pieces grow together with their master but that is only for UNUSED evil pieces and it only affects the capacity of how much potential/power that piece can reincarnate.
 * Example: If Rias met Rose in Volume 1, she wouldn't be able to reincarnate her with just 1 Rook.

Issei's regular Pawn pieces mutating is a very special case. It was because Ajuka modified the codes of his pieces because:
 * the codes were messed up Juggernaut Drive
 * for Issei to be able to accomodate Ddraig's power more.

Like what we already said, there are only 15 pieces and a King piece is not part of that.
 * Tannin has a Queen piece inside of him. When he became a King, he was given 15 pieces (Q->P). There is no King piece that was placed inside him and only the Queen piece given to him by Mephisto is there.