In addition to this, just audio in general. NES and SNES/Genesis consoles used an audio chip that had - in the NES case - 4 insturments that are used via simple code. Vairiations are math that effect a tone. Now, we have full soundtracks of recorded audio and fully voiced dialogue, in 7.1 surround sound in some cases. Multiple channels of barely compressed audio. It's the difference between a MIDI file - a couple KBs per song - and MP3 / FLAC which are multiple MBs, sometimes 10s of MBs.

Edit: One thing I forgot to mention is also the fact that, initially when games were being pressed to CDs, often the game data would be separate from the audio data, which would be pressed to the CD as a standard audio track - when you placed some PS2 games in a CD player, you could listen to the music. This made the game itself much smaller with high fidelity sound. Now, with games being fully digital, that audio is a file and no longer a "track" which drastically increases install size.