Yes, I remembered that some MUDs touted much more rooms. I've found Aarchon (15K) [1] and SlothMUD (23K) [2]. And that's just the number of rooms, the other numbers (8K mobs, 7K items, 12K NPCs) are massive as well.

But big MUDs usually have "builder" teams so the comparison is unfair. However, these numbers have hardly been matched by game studios - WoW or Runescape comes to mind. And then there's Dwarf Fortress which reaches the infinite in some categories thanks to procedural generation.

[1] https://www.aarchonmud.com/arc/features

[2] https://www.mudportal.com/listings/by-genre/hack-slash/item/...

Yah, aardwolf (the one o played in ancient times) apparently has 35,000 rooms.

Collaborative building over decades adds up!