Since I started GMing I always enjoyed looking at large/many-roomed Dungeons or modules like Undermountain, but for the life of me I never understood how people ran them, in general, but even more so if you are running a more lethal/OSR/risky, XP for Gold style of game where players are more careful, deliberate and slower. My players and I like a good dungeon a lot! But our dungeons tend to be small and even with 3 to 5 rooms and one instance of combat a dungeon can take hours.
The problems I always had with dungeons of more than 5 and certainly more than 10 rooms:
They could become one-note/boring. The almost always take the whole session, sometimes several. After two hours much less the 10th or 30th room it becomes very hard to come up with an engaging reason for the players to press on and want to explore another 5 rooms of spend entire session only in a dungeon. This is easily the top reason why I have rarely run dungeons of more than 5 rooms per session. Of course PCs could leave and come back, but that is often not very satisfying or inefficient time wise. Adding a bunch of tiny or especially empty (ish) rooms to pad the size is also not really fun for either.
Limited in scope/options for players and GM. While dungeon delving (and related combat) are a huge parts of D&D a lot of other crucial, fun things can’t be done or not nearly as easily whenever you are down in a dungeon. For instance exploring different little natural areas or biomes, structures and small settlements of the world, meeting and interacting with more than 1 or 2 NPCs, buying and selling/trading goods, intrigue/factions, politics, going to a tavern all of that and more is impossible or at least much harder and rather improbable while you are exploring a dungeon. You are not going to mingle with crowds of people or interact with 3 or more NPCs in the average dungeon. It is just not that logical or realistic. So by spending the whole session (mine are 3.5 hours) or even several sessions in a dungeon you mostly give up all those other styles or parts of the game for that duration.
I am guessing this could be somewhat mitigated, I liked how Ultima Underworld had some factions or permanent inhabitants, but even there you spend the majority of your time just going from room to tunnel to room, alone (with your party) and not really feeling part of “society”.
To put it another way, every big city will have some small or big tunnels, sewer systems and dungeons (underneath castles or large churches) you can explore. A small or big dungeon will rarely have a city above it or very close by and even if it did, the players would still have to get all those cities before they could explore the dungeons. A lot of dungeons are “stand-alone” or rather far from cities.
So is less actually more? Are big dungeons holdovers for when D&D first started and was more of a strategy/tactical game and hence more focused on Dungeons (it is in the name) ? How do you and other GMs mitigate my two issues?