Well, it isn't so much the super-specific assets that take up so much space, as there usually aren't a massive amount (due to dev time). A character's (for example) full build itself may come to a decent size when you consider how many there may be, but each character isn't made from completely individual parts. The things that take up a the most space are often shared between characters. In modern (open world) games, you won't really have a lot of individual textures for each body, you'll have a set of various skin/hair/eye textures and clothes that are used across all characters. The 3D geometry and how those pieces are used changes, but those pieces in the first place are what take up the majority of space, so you can make a lot of extra, unique content out of those assets without having to use a ton more space.

The same goes for things like the landscape. Grass for example probably has x amount of textures, but those will be used throughout the entire game meaning most of the visual landscape is using a relatively small amount of data (still a lot, but not as if 100 more feet is all more data taken up). Wind Waker's ocean may seem vast, but it's actually using nearly no space whatsoever. They could make it expand almost infinitely, if it were just the water (or ground).

From that base of assets, you can create tons without taking up much more space. Super specific things still need to be drawn up from scratch and use their respective amount of space, yes, but the vast majority of data comes from a large base of assets which (along with sound, namely non-MIDI BGM and voice acting) are what make the biggest chunk, not things like extra/altered world space or character models. This is why full games (let's use Witcher 3 as an example) can be around 30 gigs, while an expansion featuring a ton of new characters/enemies/worlds may only be 5-10 gigs (if that).

I understand your point about reskinning, but if it's ever going to happen, it's going to have to be along the lines of a "reskin". That's why the Dark World is just the Light World with a different color palette, altered NPCs and slightly rearranged world. Making two completely different, full-fledged worlds is nearly out of the picture in general because it'd be almost enough for two full games.

I wouldn't really see it as a simple re-skin if it fulfills a different function/feeling purpose, though. Also note that this is referring mostly to open world games, not games with detailed inch-by-inch progression.