I think its a little of both?
The original continents are HUGE and their quests/lore built upon by years of the original 3 warcraft games. So they felt not just physically large but like they had narrative weight. "oh shit, this is the Dustwallow Marsh, THATs Karazhan!, holy crap this is where Grom killed Mannoroth"... all that felt meaningful and somewhat legendary and the established lore kinda tied everything together so it actually felt like a complete world, rather than just a map.
Combine that with the vanilla/TBC questing design where you'd do a small little hub in a zone but then not be high enough level to do the next, so you'd have to dip out... come back... dip out... come back etc making it take a long while to "finish" zones making them all feel a bit bigger just because you didnt fly through them in an afternoon.
*gets out a soapbox*
And you also didnt fucking have flying mounts back then so you couldnt just scoot over the world and had to actually run through it so you saw more of it and felt like you were a part of it goddammit fuck flying mounts its the dumbest shit in the world all my homies hate flying mounts.