I can't see how its 21gb.
Is it ship textures?
Audio files?
cut scenes?
World of Warcraft, 10 year old game, with large amounts of content is about 25gb currently
EVE, mentioned above, is 20. It has 277 different ships, and I am not sure about Stations, Defenses, jumpgates, or NPC's.
It really is hard to comprehend. Can you datamine it? hell, just folder sizes would be fine