The article linked says theyve set their goal to make the game 25% bigger (as in size) than the first one because series s has 25% more memory than old gen consoles. At least according to the writer of the article, couldnt find any video of the presentation itself, but he mentions it was said by multiple developers so its very unlikely he misinterpreted it, but then again he also says the game is supposed to be "much bigger" that the first one (not specified in what ways, but he goes on to talk about the amount of voice/text and the bugs right after) earlier in the article, safe to say the "much bigger" relates to the amount of detail/content while the 25% is about the available map space.
I am not surprised there is confusion since bigger can only relate to size of something, while he has freely exchanged the phrase "more detailed" with "much bigger" just few sentences earlier, the article is not well written.
While theres 2 maps, the 25% relates to the combined player accessible (playable) space: