As i said, if Archimonde could be proven to have greater mind, or Killy a greater mind than the Lich King, then perhaps they would have more power, his magical powers were indeed expanded since Kiljaeden gave the Lich King more power overall did he not? he doesnt use Shaman powers, he must have gained lich power and other such from Killy.
Becci or you once said that Killy and Archi have potential omnipotence, their understanding however of power and such is not vast enough to use omnipotence and likely never will, so i was saying if Lich King has a greater mind and is expanded beyond them, perhaps he has a few more tricks than those who cannot see as far as he can in the mind.
It simply grew full stop, it doesnt say "just so" he can control the undead, thats definatley an ability of his mind but that is never stated the limit, the mind in a way can amount of intelligence if your mind is so much vaster than others.