Vast spaces and imposing structures are awesome but aren't much fun to play in. Most games seem to solve this by making such spaces/structures occupy the backdrop and/or keep parts out of reach of the player (like the cliffs from half-life
, for an extreme example). The worlds of dark souls and bloodbourne seem impossibly vast and full of towering buildings, but remove anything that isn't a space the player can occupy and you're left with a much smaller area tailored for gameplay.
Scale is probably the single hardest thing to get right for me and is why I greybox everything and don't even think about art until the space is correct. In my test map, cartographer's nightmare
I tried to make spaces feel large while limiting the play space. For instance, the largest space isn't even playable and is first seen by the player while he is in one of the tightest spaces. Other examples include being able to see multiple stories while being confined to a tight footpath, and an 'infinite' ceiling forever out of the player's reach. Despite these tricks, one of the criticisms I still remember is that the spaces were too claustrophobic and the player felt like he was always about to "hit his head". Go figure!
tldr: scale is one of the most important factors in gameplay and therefore one of the most difficult to get right.