If you think of table top rpg's as a DIY hobby, then you mostly make your own dungeons, and in that case I doubt much of anyone just sits down and creates a megadungeon. You sit down and START a megadungeon, but it spends a few weeks as a little dungeon with some unfinished passages, and after a year or so it is a medium sized dungeon with some unfinished routes to greater depths, and a few years after that it is a megadungeon.

Quote from: Haffrung;1026102It's interesting that the current zeitgeist around dungeons (judging by published adventures and forum topics) are small dungeons (15 or fewer rooms) peppered through a larger adventure path or sandbox setting

That's my preference because then it makes sense for them to all be (mostly) one faction.  If there are only a few rooms, then PCs busting through and taking them down piecemeal makes sense.  One of my world-building pet peeves is the massive dungeon of faction X where virtually none of the various rooms of foes make any attempt to help one-another out and let the PCs go from room to room and take them down piecemeal.

Now - you can come up with in-setting reasons for the bigger dungeons, but they tend to feel like rationalizations more often than not.

It's the point of the game in D&D, so yeah I have dungeons a plenty. Big ones. Small ones. Ones you go down. Ones you go up. Ones that are indoors. Ones that are outdoors. All of the lore and other arcana in my setting is found/answered in the dungeons (because knowledge is treasure, and treasure is in dungeons).

I have quite a few, but they tend to be smaller dungeons (a crypt here, an ancient temple or a secret chamber complex beneath a manor there). In my current campaign manuals are often found inside dungeon-like areas (and the players are often chasing after them).

For the most part, a significant portion of my games are and have always been explore an unknown environment, interacting with environmental challenges, 'traps', 'opponents'/'potential allies,' and seeking various forms of treasure. Whether everything qualifies as a dungeon or not is often up to interpretation. Many times the thing keeping both players and monsters inside the 'dungeon' are not walls, but the fact that that is where the objects of interest happen to be. But since I mostly play D&D or analogues, it really helps to have choke-points and the like, so walls or the equivalent are common as well. Since (re-)discovering hillforts being a thing, I have made a campaign where entire careers are spent delving mostly through above-ground dungeons which still have topology and effective walls, just not ceilings (and then the occasional cairn, catacomb, mine or monastery).

I adopted the concept of living dungeons from 13th Age. My design is generally informed by Dungeons, Dungeon Keeper, Overlord: Raising Hell and Malazan. Every dungeon is centered on a dungeon core/heart and produces or summons the architecture and creatures it requires. Why the dungeon exists and what it wants, if anything, is variable.

Dungeons-style dungeons live to lure adventurers and heroes in order to feed on their satisfaction at overcoming the challenge of the dungeon. The danger to the dungeon comes from the dungeon needing to balance between fattening the hero and having enough forces left over to kill and eat them.

Dungeon Keeper-style dungeons and Overlord-style dark towers are essentially RTS-style bases that expand in an attempt to conquer the land. To defeat them utterly requires killing the dungeon keeper/overlord and breaking the dungeon core/tower heart.

Malazan-style dungeons are prisons for powerful archfiends, archons, etc.

As in ACKS, PC wizards may build dungeon cores in order to lure/recruit mystical creatures for spell components.

As in the cartoon Tower of Druaga, shantytowns may appear around some dungeons with economies based on adventurers' guilds farming the less dangerous levels in a satire of MMOs.

Dungeons are everywhere under the ground ... or maybe it's just one dungeon?

Currently? Not very prevalent. Once every many sessions.