The beauty of procedural generation is that you can build up solar systems on-the-fly, so all you have to save are the changes made to that solar system, which can be massively reduced via clever algorithms (which I'm sure infinityadmin is full of). The only thing that takes space are the (in infinity's case) 6 base planet textures, the asteroid textures, the ship models, the base buildings for the cities (Which are also procedurally generated), the space stations, among a few other things. Nebulae and a large amount of background prettiness is all procedurally generated. Spore has nothing on this game.
EDIT: not counting music, sound effects, GUI images, and the like.