So I just installed Witcher 3 to have a gander at the next gen improvements and noticed a rather large
size discrepancy between the install folder (49 GB - that was also the number on the download/install)
and the offline installer (79 GB).
The game says both expansions are installed...so what's with the 30 missing GB?
Even if the DLC install to C:\user the download still said 49 GB.

maybe is for the other languages voices

Good thinking....

Yes, it's about the languages. But if you think that's heavy, check Cyberpunk and Shadow of Mordor.

And yes, they could do it differently of course (language packs), but sadly that's the way they decided to go.

Let's just be glad they don't install all languages in any case, like it's done in other games.