As mentioned, all the game is 3D models.
I actually looked around and honestly the voice folder is only 6GB - quite a bit less of the overall game's footprint than I'd expected. Just a ton of high quality, high resolution art assets (maps, effects, models and animation, etc.) I would imagine make up the vast bulk of the required size.
Only the Characters are 3d models. At least in this backer update:
https://youtu.be/oiep0wYfQPQ?t=38
I actually meant "all the game's actors".
Sorry, I tought you meant all ingame scenario. But anyway, games like The Witcher 3 are full 3d and their weight is 30gb, so don´t know what is so heavy in the game.