Undercaverns isn't too big, what the hell are you talking about lol ?
Did you ever look at the emberflow map ?
It's AT LEAST twice bigger.
While emberflow might be longer from point A to point B it feels different because its all connected by small tight passage ways from one point to another. Caverns is different since its a larger area that connects to another large area which then connects to another large area ect ect. Im pretty sure thats what he ment by it, i could be wrong.