I like big dungeons.
They aren't even vaguley difficult to justify and it is silly when folks mention they are but then play in campaign settings with dozens of underground dwelling races. The dungeons are where the goblins, orcs, drow, dwarves, derro, druegar ,and so on, dwell or dwelled.
Level by level travel shouldn't be a trick or gimmick for main levels of a big dungeon, but sub-levels work fine with such a gimmick.
The larger a big dungeon is the more playable it is. No one would try to clear out a city room by room so why try that with a city sized dungeon? Themed section and sub-levels that ARE tricky to get at or obviously different from common areas of the dungeon should be where all the real action and loot are.
There are plenty of multi-level dungeons that humans constructed in the real world, some are salt mines, catacombs beneath cities or deep undrground redoubts with churches, wells and grain storage bins deep under the earth. Our own world has an unknown number of "big dungeons" and we don't tend to all be morlocks and vacation in pelucidar, so why not dozens in a fantasy world?
Big ruins are neato as well. That way you can get multiple dungeons and indoor and outdoor adventuring all in one general spot.