Yeah, the megadungeon ("campaign-dungeon" is an alternative term I tried to introduce that didn't gain much traction -- emphasizing that the nature rather than the size of the dungeon is the important distinction) is a way to "sandboxify" the dungeon concept, blowing up the size and scope of the dungeon to the point that it becomes essentially a campaign-setting, serving more or less the same function as the wilderness hex-map serves in a more "traditional" sandbox setup. In a mega/campaign-dungeon there are essentially "points of interest" scattered throughout the dungeon plus lots of "dead space" (empty rooms and miscellaneous tricks, traps, monsters and treasures) in-between. The players have their choice of which points of interest they wish to seek out and explore (with dungeon-levels allowing them to assess likely difficulty and reward), or if they choose to they can grind in the dead space, or they can leave the dungeon entirely and have sandboxy adventures in the wilderness or in town (the idea shouldn't be that the megadungeon is the only place to have adventures, just that it's the best and most convenient, especially for low level characters).
Two important keys are that there are always more points of interest (the party should never feel "stuck" exploring a particular area because there aren't any other options), and that the dead-space is never "cleared" (there will always be at least vermin and wandering monsters there). Certain areas within the dungeon will eventually become familiar -- the players will return to and pass through them again and again on their expeditions, eventually developing detailed maps, and even later not needing those maps because they've got the area memorized -- but around the edges (symbolic, not necessarily literal - depending on how you draw your maps it's entirely possible to have "remote" or hard-to-access areas right in the middle of the map) and deeper down there will always be fresh and unexplored areas.