I mean yeah if you take the Mushroom-like landmass that is the Mushroom Kingdom and put every single location that has been confirmed to be part of it in there, that would show that what Mario Odyssey showed us from it was just a tiny percentage of the entire kingdom.
In my headcanon, many of the locations of Mario 64 are in the Mushroom Kingdom. Bowsers castle is all the way past the ocean in his Kingdom, which is also bigger than what Odyssey showed us. There is a town for Koopas to live in, Bowsers castle, and that lava area thats in almost every other Mario game. Only, you dont explore all those in Mario Odyssey, because your goal is the castle.
In fact, all the kingdoms are way bigger than what you actually get to explore from them. In the background of Fossil Falls, there are still other similar-looking islands, which means theyre all part of the cascade kingdom, but the only area from this kingdom you get to explore is Fossil Falls. Same goes with Tostarena and Steam Gardens and especially Crumbleden, because even if its one of the smallest levels in the game, the Ruined Kingdom is that gigantic batwing-like landmass, so it has so much more into it than it seems.