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There are alot of PC games out there that have (seemingly) roughly the same graphics (with bump-maps, shaders, etc), sound complexity and play-through time yet one of them needs 4GB space while the other 13GB.

Why is that? Why to they differ so much in size?

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That's a little like saying i have 2 images that are both roughly the same, why is one bigger than the other. It could be that one's a png and the other is a jpeg. i.e. The end result might look similar, but data used to get to that end result can be very different. –  George Duckett Dec 13 '11 at 14:19
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What specific problem are you trying to solve with this question? –  Tetrad Dec 13 '11 at 16:09
    
As an example of extreme small space check out 64k scene demos at pouet.net –  bobobobo Dec 14 '11 at 1:20
    
It really does bug me that disk space is treated as a commodity by most major developers. CPUs and memory have grown in power/size at a massive rate, whilst storage has not kept up at all. Even trivial compression of bitmaps could save 20% of my disk at negligable load-time overhead. –  Polynomial Dec 14 '11 at 11:37
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@Polynomial: you have that backwards. Disk space has grown far more rapidly than memory in consumer PCs. Since I got my first PC in 1990, RAM has increased by 3 factors of 10 and disk space has increased by 5 factors of 10. If RAM had grown that fast my PC would now have at least 100GB of it. The average user is more concerned with performance than storage use. Ask someone how big a game is and most people will shrug. Ask them how long it takes to load and you'll usually get a definitive answer. (Also, disk space is a commodity, but that's irrelevant: so is RAM). –  Igby Largeman Dec 14 '11 at 18:12
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Most of the size used by any game lies in its assets, most notably, audio, video and (to a slightly lesser degree) images/textures. Binaries and game data is typically much smaller than any of those.

Then, all of these assets may or may not be compressed, may use different compression schemes, may have different formats, and even have different resolution/bitrates.

One game may for instance have large resolution HDR cubemaps while the other uses normal textures. One game may store shadow maps for each environment while the other achieves the same effect by computing the shadow map at runtime. One 2D game may use skeleton animation while the other has the exact same visual output but storing each animation frame individually.

It's pretty easy for two games to appear to have the same order of complexity, when it's not necessarily the case from the asset point of view.

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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:

  1. 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.
  2. 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 :)
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This is a great answer. The trade-off between speed and size. I would extend this example a bit - for example, games that never see a loading screen would probably have decompressed assets from the start since they can't afford to clog up the CPU and GPU decompressing while playing; on the other hand, those with loading screens can decompress assets then and result in a smaller file size. –  DMan Dec 13 '11 at 22:08
    
@DMan: I added some examples of the games that load assets in compressed form to my post to address your suggestion. Thank you a lot, it was a really good idea! –  Alexander Galkin Dec 13 '11 at 22:46
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I think the kind of data with the most commonly underestimated size is audio, especially music and voice recordings. This is because audio compression is hard, audio mixing is tricky, and while the eye can tolerate pixelated or blurry textures, level of detail popping, and a few missed frames here and there, the ear is absolutely unforgiving with audio stuttering or clicks.

That said, videos can often make for the most storage a game will use.

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this is a bit of a subjective question. but still, if you look at the install directory of these games you will straight away be able to see the differences. at a guess the 13gb one will have several gb of movies and audio files.

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It's sometimes because of "piracy" reasons. They add padding-files to games so that you have less incentive to illegally download them. If you really want the game in question and pay for it, you will go through the trouble of buying it digitally even though the file is large.

It's also a way for the companie(s) to measure how much interest the IP or franchise still has;

"Do they still want to buy or pirate this game even though it's a very short game yet very cinematic but is about the size of half a bluray-disc?".

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