IMO ideally PS3 games would install less than 1GB of data onto the harddrive (excluding patches) for as fast as possible bootup (starting loading music and intros) and from there on start streaming everything from harddrive cache/Blu-Ray.
But devs seem to have the freedom to do whatever they want on the PS3, so many multi-platofrm companies will cut corners if they can get away with this (cheaper).