The Balrog vs The Witch King

[Boromir88]

Why does the breaking of the Gate prove an increase in the Witch-King's power? The Witch-King was already a sorcerer:

Those who used the nine rings became might in their day, kings, sorcerers, and warriors of old.~The Silmarillion; Of the Rings of Power and the Third Age

Also:

King of Angmar long ago Sorcerer, Ringwraith, Lord of the Nazgul, a spear of terror in the hand of Sauron, shadow of despair.~(Gandalf to Pippin) The Siege of Gondor

You don't think all those pretty lights on Amon Sul was all Gandalf now do you? The Witch-King (and most likely the other Ring-wraiths) became 'kings, sorcerers, and warriors of old' because of the use of their nine rings. Just because the Witch-King lights his sword and breaks down the gate doesn't mean he has had an increase in power as he already was a sorcerer with some neat tricks.