My thoughts are that since there are 3 years and 3 months separating the release of Skyrim and the estimated release of W3, it's therefore acceptable to say that being only 20% bigger isn't exactly as groundbreaking as one might wish, and that it makes sense to hope for something more awe-inspiring. With over 3 years separating, I think most gamers expect a much bigger improvement than "merely" 20% bigger, and maybe CDPR also wants to set a higher standard. Maybe.
Just to clarify: I realize that even a map that's 20% smaller than Skyrim's can feel better, since it all depends on what you fill the area with and how, and I know that all talk about the map turning out to be much bigger than previously announced is merely speculation, without any official confirmation.
Personally, I think you'd have to be a wizard to make something as gigantic as the calculations here are hinting at, while still keeping the top-notch quality. Not that I don't want to be proven wrong, but I'm (very) skeptical.