Just in addition to what David already said in his answer:
It is true, that up to 95-99% of almost every modern game size is constituted by its assets: video, audio, textures etc.
If you are wondering why some games take only 4Gb whereas others much more having same level of graphics?? It can be also due to the fact that some game store their assets on the hard drive in compressed form and just uncompress those on-the-fly during loading. This adds some overhead during loading, but same time the total size of the loaded data remains relatively small, since the data is first read compressed and only decompressed as soon as the reading is complete.
Other games decompress their assets on your hard drives completely and load resources already in "final" format. This are normally games where the installer size is much lower than the size of your installation. Some resources are so densely packed using some proprietorial algorithms that decompressing results in files being 10 times larger. In this case the assets are stored in non-compressed form and loaded directly into the memory. Here the throughput of your drive can become a crucial point, but there is no additional overhead due to on-the-fly decompression.
Both approaches have their pros and contras: the storing in compressed form is essential if you can't install the game on a re-writable medium and has to read it from ROM, like DVD- or BR-ROMs (this is often the case for game consoles) here you must do the decompression on the fly. PCs always have a writable drive (HDD, SSD etc.) so the decompressing in advance is quite common here.
The complete decompression of game assets can improve game performance allowing it to be started and played even on old computers having not so powerful CPU or GPU. Game consoles usually have a standardized hardware you can easily test your game on to see if the performance of your game is fine and do some other tricks (like down-sampling your sprites etc.) to reach the necessary performance.
[EDIT]: How to recognize if the game is uncompressing its assets on loading? Here are some symptoms you might have noticed at some games:
The game offers you some way of interactivity while loading data.
- In Mirror Edge version for iPad, this has only 123 Mb you are presented the radio message from the resistance radio in the form of the floating text á la Star Wars intro. It normally takes you for 20-30 secs to read the message before a new mission starts.
- In Anthill this is only 17.1Mb on iPad the loading screen generates funny messages, something like "We are polishing the riffles" etc.
The game mimics the "booting" of a normal computer system, giving you some information about the progress of booting.
- A classical example here is the World of Goo, which boots your "Corporation of Goo" computer every time you start it. Again, it is just 80 Mb large -- almost nothing :)