Skyrim was just the most popular. It wasn't the biggest open world game by far, nor was it the most dense RPG even in the generation it was in. It was popular, and easy to play.
Being popular is probably why it's used as a point of comparison though: it's something most people are familiar with. And it was also dense with content, even if that content was (for many people, like myself) repetitive and uninteresting.
Personally I'm less interesting in
size so much as
density and
variety. Size of map isn't really an issue with modern games, but demands for bigger game worlds don't take into account the huge increase in development resources. End of the day, if people want a game world that's dense with content that feels hand tailored and interesting, not just in quest design but also topography and presentation, concessions need to be made. More or less every insanely huge game map/world that has been made, or is being made (see: No Man's Sky) relies heavily on procedural generation. As dev tools become easier, and procedural generation algorithms more complex and accurate, in theory bigger game worlds with the sense of being hand tailored can exist. But for now the logistics of manpower and money versus time required to hand tailor a world need to be balanced.
And yeah, I think that's why Skyrim is used as a point of reference. It's a huge game world, hand tailored, with a lot of content (criticisms of said tailoring and content aside). And it's well known. Wild Hunt has huge landmasses, hand tailored, with the promise of a lot of content. The advantages of building specifically for current generation hardware are evident in the presentation and asset density, but to me the more impressive part will be how well they've accomplished those hand tailored quests and locations relative to the size of the maps. People making stuff, instead of algorithms making stuff.
EDIT: As a side, I also think scale/size of the map does spark potential for delivering on atmosphere within the intended setting, which is also a goal for these kinds of games. Sailing between two islands tends to be more immersing and convincing if the distance is somewhat believable instead of aggressively condensed. Same with forests, fields, and the middle of nowhere. Not everybody might like it, but if the size of the map allows for you to have gorgeous vistas over fields, or get lost in a dense forest, when the boarders are not painfully obvious and tiny, it's easier to get immersed in the adventure.
Wind Waker is a good example for me. It's my least favourite 3D Zelda as a whole, but I quite like the perceived emptiness of the ocean and sailing between islands. The combination of emptiness and distance immerse me in the sailing adventure and excitement of finding a new location. If it were more condensed that immersion would be reduced.
