How do they fit in those cramped tunnels? You can't just build mines to giant scale. The earth wouldn't take it, and would just collapse in on itself
You should visit a Marble Mine in Kansas City (I played in a Paintball field set up in one) or See the Hutchinson Salt Mines....
Okay the technology is newer on those.
So lets look at the Salt Cathedrals in Poland that were started in the twelfth century. But I don't now how long after that they got so freaking huge.
I think the earth can handle it

However I agree the Kazad should get a mine they can build there (would their workers have to walk on mountains to do that? I guess if just workers not a big deal) Jotnar... hmmm maybe a reduced production Wind Mill. Beats a useless spot in their limited space.