How much of the space on some of those larger ones is simply from not bothering to compress various assets(audio/cinematics/etc) because they had a load of excess space to use?
Depends on the asset. unlike what a lot of gamers think. textures are not stored uncompressed on disk, EVER, unless the developer is a complete utter moron. since all graphics cards for the last... 10 years or so, has supported compressed textures in memory. and even with 2GB graphics cards, textures are so huge, you need to have them compressed to fit all those huge size textures. So the only thing you do by storing uncompressed textures, is increased load times, increased GPU load by havign to live compress the textures, unknown texture quality, as the GPU will set the quality, whereas pre-compresed textures the artists have full control of the quality of the compressed textures and and make sure it doesn't have errors or artifacts or lost details.
for Audio it's a bit hit and miss. but then you have people who think that all the in game audio is stored in pre recorder 5 channel files as well, when it's in fact stored as one channel files and the surround is live calculated by the engine.
what usually takes up a lot of space on disks is pre-rendered FMV's which is usually a JRPG thing. though games have proven that FMV's are bettered done in game engine nowadays, and that'll be even more so in next gen.