'Biggest' game I have is Baldur's Gate 3, which is about 140 GB, so really just about seven times as big. That's quite good, considering how much game that is!

Not at all. For example, Rimworld saves all the map and world data in one big XML (which is bad btw, don't do that): about 2 million lines @75 MB, for a 30-pawns mid-game colony.

So you see, Data is not what uses space. But what uses space instead is, if you don't properly re-use objects/textures (so called "assets"), or even copy and repack the same assets per level/map, because that saves dev time.

Ark Survival Evolved, with "only" about 100 GB requirement, was known as a unoptimized mess back then.

Witcher 3 mod "HD Reworked Next-Gen" has barely 20 GB with 4k textures and high-res meshes. And you can't say that Witcher 3 is not a vibrant and big open world game.