Ovinomancer
No flips for you!
I tend to disagree. It is entirely possible the whole of the explorable world is a single dungeon. Indeed there are games with that premise. In that case, I'd like to think you'd agree that the dungeon design is world-building. World-building occurs whenever an element is added that potentially affects more than a single locale. The king of a realm may reside in a dungeon equivalent. Defining the king's personality, motivations, and capabilities is world-building even if he is placed at a specific locale.
I think the line between locale construction and world-building is an artificial one. The locale exists inside the world and needs to conform to any precepts already established. Its design may suggest new precepts to be added to the world. Determining that the cult of <fill in the blank> always has at least three separate entrances for all major buildings is world-building even if the decision comes about because of the design of the first building as part of a locale.
That said, I agree every wall and door placement in a locale is not world-building.
No, I would not agree that the design of the differing parts (or the whole) of the dungeon is the worldbuilding. It may be so, but is not necessarily so. To me, the world building started in the development of the premise that the entirety of the explorable world is this dungeon and the necessary co-assumptions this entails. The actual windings of the dungeon are details based on that premise, and could be authored prior to play (DM driven) or during play (player driven).
Now, I will stipulate that once those details of the dungeon are established, they become part of the worldbuilding -- ie, they are now established parts of the world that future play is expected to respect. But, when you build the world sized dungeon, you've already decided on the premise that the world is the dungeon. I would go so far as to say this holds even if your reverse the creation process -- if you built a dungeon and then decided that it was the world, the world building aspects as presented to your players is that the entire explorable world is a dungeon. That's the hook the game turns on, to mix metaphors.