You have to realize that different types of files take up radically different amounts of storage space.
Things like the actual game code and mechanics and scripting and all that stuff that really matters to a game? Yeah, that's the most densely compressed data you can find.
The stuff that takes up the most storage space? Movie sequences, followed up by other visual media, like textures, then audio media, like background music and voice acting.
For an extreme example, look at Dwarf Fortress - basically no visual media at all, and only one song that takes up basically half the file size of the whole game. The actual file you need to download to play the game is less than 10 megs... and then a single save file can take up a hundred megs, thanks to all the procedurally-generated data.
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EDIT:
Mass Effect 1 and 2 (and probably GTA 4, don't know, never played it) have vast amounts of dialogue files for all the voiced-character cutscenes. Yes, there's voiced dialogue in the Beth games, but not nearly on the same level.
If the difference is honestly 15 gigs of voice acting, then BioWare must be allergic to compressed audio files.