There's a thing about compression vs quantity. If you have lots of uncompressed things (textures, sounds), uncompressed, it will take HUGE amounts of space. If you have few compressed things (sounds and textures again), it will take hardly any space. And then there are procedurally generated things (see the florae in Oblivion) - which are simply generated by the CPU when needed.
Size of storage for games has nothing to do with game's lengths - games aren't linear medias like songs or movies.
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