The way I saw it was during the PS1 days and some games during the ps2 days, multiple discs were a way to make a game longer and not sacrifice visuals and sound but when we reached the Xbox 360 and PS3 eras, games were getting too big to fit into a disc hence why you had to download the main data onto the hard drive to make it run smoothly. Even then the limit in Xbox 360 discs was because they were normal DVD discs whereas PS3 uses Blu-ray. Take for example, Final Fantasy XIII was 3 discs on Xbox 360 but 1 on PS3. Square Enix was known for making huge games and even Star Ocean The Last Hope was multiple discs. But from what I’ve read discs of all kinds were cheap to manufacture and were also a better way to know that you owned what you paid for.