Megadungeons don’t just need huge number of rooms, doors, and corridors, they need vast spaces. Even a maze of rooms and halls is in totality a vast space. Describing and making use of these vast spaces is a challenge for the DM.
Just how big a vast space is a “vast space”? A vast space should be identifiable as unique onto itself in some manner and link other locations. Such a space can serve as a hub to link together sub levels and even smaller areas that might otherwise be considered their own dungeons. There is no one answer but when it’s being enough the size and reach become a feature it’s likely vast enough to qualify.
A few examples as outline:
The Huge Cavern, a huge cavern that forces a change in mapping scale or even means of description. If the place is big enough to fit a skyscraper and a multiplicity of subterranean features you can not see in but moments you’ve got a vast space. There are some truly huge caverns in the real world here’s but one: http://ngm.nationalgeographic.com/2011/01/largest-cave/peter-photography, good luck mapping that out in 10’ x 10’ squares.
The Chasm/Canyon, at first this is just like a huge cavern but it enforces a separation from point A to point B and forces people to think about the third dimension that they likely don’t want to fall into or get lost in. Sure you can explore the Chasm or Canyon but most of all you want to get to the other side.
The Well of Worlds, A great hole/shaft/stair going down it may or may not actually link portals between worlds but it does link hugely separated areas within the megadungeon that should feel like one has travelled from one domain to another.
The Dark Lake/Sea, a vast underground body of water (or other liquid) breaks the normal mode of travel, you can’t just walk across the expanse different modes of travel an action must be considered. the real action may be beneath the surface of the water or isolated on lonely islands that are invisible in the darkness. Can one sail in the still waters of the dark sea? Is there a current? Is the party equipped to deal with aquatic hazards?
The Lost (or not so lost) City, a city space that is clearly contained stands apart from the rest of a megadungeon with the needs of occurrent or former occupants begin visible. Folks live(d) here and their needs were met, everything you could expect to find on a surface city and more may be here. Roaming about an kicking in doors to pillage everything beyond may prove an unwise strategy in a such a lost city where the occupants should be more familiar with the surroundings and have more routes of travel than are typically found in a dungeon.
The Pits of Toil/Forges of Might, a vast mine or industrial space geared to it’s own purpose will set a vast space apart from a collection of dimly bigger rooms. A factory is a very different environment from a civic center and that should be the scope of difference between the pits and other dungeon spaces.
The Monster Coliseum, huge gathering place geared to sport and combat can have multiple layers of use and focus for a megadungeon campaign form prison breaks of unwilling gladiators to exciting competition on the arena floor and possibilities to mingle and pickpocket among the curious crowd of onlookers. A Coliseum is a great environment to place challenges to meet characters of all different skill levels and offers more then the daunting challenge of mapping a vast space.
The Deep Wilds, similar to the huge cavern but the locations and behaviors of local life become more important and the prime feature in forest of roots and/or fungus. The comings and going of the local fauna and peculiarities of indigenous flora are far more important then mapping a 50’ stretch of space.
The crossroads or highway, an obvious travel route much much larger than a normal corridor is a vast space underground. Perhaps instead of a huge corridor it is an aqueduct or canal. It’s just to big to control the entirety of by a stationary foe and encourages much more mobility than is atypical in a dungeon environment and offers clear routes of travel.
These vast spaces can serve as stairway between levels, or a clear threshold separating low level, midlevel and high level play.A dungeon that contains or links one or more of the examples above can’t help but feel mega to the players in a megadungeon campaign.
A DM has to consider visibility, mobility, and even noise differently in the vast spaces here. A typical dungeon fight seldom involves more than a couple chambers of foes but with the ability to travel more and be noted from a wider array of potential foes the tactics used by player and descriptions of the DM may have to vary and the Megadungeon building and runnign DM should keep in mind and anticipate the differences in scale and scope.