I'm going to have to disagree and give a counterpoint why you should never design dungeons like X/XIII or "Tube world" as they should be called.
The reason I like XII's dungeon/world design better is that the scale for me adds a ton more immersion cause its actually feels real. You get to explore these massive lands and monolithic dungeons and by god do they feel massive and monolithic like they are described and appear to be to the player. I can't get that out of the the other two titles, sure they are pretty but it soon becomes quickly apparent you are caged in on a simple road and all the beauty is simply just "wallpaper". I can't tell you how many times I was trying to explore in XIII's world and wishing I was off in the distance cause it felt like something more interesting was going on over there.
The spell of immersion easily smashed when you walk over to a place that should be easily accessible from sight (worse in XIII since your party can jump around structures) but find out you can't cause its not real, its just the wallpaper decorating the road. Its pretty, but its soulless and does nothing to help me care for the world. Getting to Gran Pulse in XIII, I never really wanted to go back to Cocoon cause GP felt so much more real than the rest of the game and I kept asking myself why the game couldn't be more like this. Even the story doesn't really justify the titles claustrophobic design, as your party is basically pursued by "boogie-men" for most of the game.
The terrible dungeon/world design has done nothing to help me feel for XIII's world, I feel FFI and FFII have far more immersive and interesting worlds than XIII has and that's sad when a twenty year old game with a town called "Elftown" is a much more immersive and fun place than a modern title using the highest scale graphics and a team 50 times larger to build it. We're in the modern age of gaming, we should be using technology to give the player more control and choice in games, not restrict them for the sake of story. I feel choice will create an immensely more immersive experience for the player over a world stripped of it with shiny wallpaper to compensate for it. One makes the player feel they are controlling and truly interacting with the world while the other makes them feel like they are on a poorly managed movie set where the lighting and raffling can be seen holding the scenery up.