Why so large? An adventure game that came out in 1997? Is it just a ton of uncompressed everything?

Pardon my ignorance.

From what I see of the how the game is made, a lot of images and sound.

I might be wrong, though.

The original disc release was on 3 CD-ROMs. Recent Gold Edition which was published exclusively on Steam is even bigger than the original one (it's 2.8 GB).

The game is big because it has plenty of cut scenes & dialogue and the concert scene has the entire Sonata in A major for Violin and Piano by César Franck.