Uncompressed audio and textures, people say it's video without actually looking at video file sizes which are usually small with a large portion of games having in game rendered videos for a lot of the parts you can't control.
Thief is 24GB almost 2GB of movie files. Maps are 11GB, not much in audio but by a guess at folder names it's got only 500MB of audio which are likely those random conversations you can overhear. Shadow or Mordor is 45GB movie folder is < 6GB, audio is < 4GB, the main folder has almost 34GB of texture asset files.
Unity is 40GB, it has over 5GB of audio files but less than 2GB of video data. 1080p pre-rendered video is not that large, it's no where near bluray quality. More and more cutscenes are done via in game rendering and just some scripting to direct characters... it's why so many in game cutscenes are buggy as hell these days. Sure it looks daft when you have some armour on and the cutscene has you wearing something else entirely, but is it any less immersive than say in Dragon Age when your party members end up standing infront of the camera blocking it, or your characters face is clipping through the collar on your coat(which happens a hell of a lot). Video really isn't the issue with increasing game size.
It probably used to be that you have graphics cards with 100-150gb/s at the top end, 20gb/s at the low end(be that console or a igp). At that stage low texture settings would require say 500mb of data installed but you also install the 1GB medium setting textures and 2GB of high setting textures.
Now with the low end and high end scaled up it's more like 3GB low settings, 6GB medium settings, 12GB high settings. Added up you end up with a vast amount of texture data. One possibility, depending on how textures are swapped out for lower IQ in distance, is to give the user the option to not install low/medium textures at all so rather than 21GB you would end up installing 12GB.
A little bit of sense in terms of intelligent install which tells you how much vram you need for which setting and to let you choose not to install whatever settings you don't want with an ingame option of should you chose a setting for which you don't have the textures installed it gives you a confirmation box saying it requires installing X GB's more from disc to enable setting. Maybe another option in game to remove texture settings as you may want to test which settings work best for you then have it be easy to free up the hdd space once you're happy with a specific setting.
In fact the game size isn't really an issue, we had 10gb installs when 500GB drives weren't that cheap, now it's 50gb installs at the top end when 4TB drives aren't that cheap.
The big issue here is that SSD's happened, if we were all spending £100 on 3-4TB hdd's then we'd laugh at 50GB requirements, but now it's £100 for 256GB SSD and with the OS and some other programs you can only have 3-4 big games on there before you're out of space and you want to keep a fair amount free for performance/lifespan of the drive.
5 years ago I probably had a couple 500GB drives in raid for OS/games and a couple of 1TB drives. Without SSD's I'd have 2x1tb drives in raid and a 4TB back up/storage/video drive, instead I hav a 256gb OS SSD, I have 2x128gb ssd's(not raided) as they were cheap to have further space for games and then a 2TB drive for downloading crap onto and storing videos/backup stuff.