Dwarven cities are always implied to be massive and sprawling lorewise.
Dark iron city extends from the very core of Blackrock Mountain (largest mountain in the EK) to its very summit.
Grim Batol is also implied to go much further down than what is seen in-game.
Alpha Ironforge had multiple levels and an element of verticality to it.
I expect Khaz Algar to be a zone-wide city, like Zuldazar. And much like Zuldazar, I expect it to have multiple levels and an element of verticality to it.