Yeah, because as we all know, the Underhive of Necromunda is such a quite place.
I'm sure cops and other security details would be keeping the peace, instead of soldiers. And who needs lots of troops, when everyone's packed in and not going anywhere? Want to escape, go ahead, live in the wastes an die. There's no point in policing the slums, let the rats fight it out. Just keep them out of the better upper parts. If the dudes were all spread out, you'd need lots of dudes to keep tabs on them, but stack them all into a small space and put a few gunmen up on a tower to shoot anyone who gets out of line, and you'll be fine.
The 5e rulebook hive world, as I remember, had an annual tithe of 2,000,000 troops. Since tithe is 1/10 of the armed forces (in general), that would mean the planet has a standing army of 20,000,000. In the 3rd war of Armageddon they raised 120 regiments of Hive Militia. Even if there were 100,000 dudes per regiment (Valhallan numbers), it's still just 12,000,000.