This shouldn't surprise anyone. GTA V was one of the few late life 360/PS3 games to come on multiple discs, 3 discs as I recall, making it one of, if not the largest, 7th gen games. R* games are huge in scope, RDR2 is said to have an even larger map than GTA V. RDR2 runs at one half 4K resolution on PS4 Pro according to digital foundry, 1920 x 2160, so we're most likely looking at bigger texture installs on Pro. Audio will be a large part of it, as I recall R* doesn't like to compress their audio files. 

I seem to remember installing 3 discs on my 360, but might have been 2. Regardless, the game was huge on last gen, 15.5 GB, and that was before GTA Online was added. GTA V had one of the biggest installs of any 7th gen game, there were only a handful of 3 disc games like Final Fantasy 13 which were larger, and GTA V ended up passing them all after GTA Online hit. It was so big that it wouldn't fit on a launch model 20 GB Xbox 360 after GTA Online hit.

Lol...What is proper compression?

Dont think you can compare lower resolution Nintendo games with upto 4k high fidelity games.

I seem to remember installing 3 discs on my 360, but might have been 2. Regardless, the game was huge on last gen, 15.5 GB, and that was before GTA Online was added. GTA V had one of the biggest installs of any 7th gen game, there were only a handful of 3 disc games like Final Fantasy 13 which were larger, and GTA V ended up passing them all after GTA Online hit. It was so big that it wouldn't fit on a launch model 20 GB Xbox 360 after GTA Online hit.

I looked it up, and it's 2 disks. Apparently the player never needed to swap disks ( good thing, that would totally ruin a game like GTA V. lol ) just install the first disk and play off the second. Actually, that 15.5gb file size range was quite common on the PS3 as far as first and third party exclusives were concerned. The 3 PS3 Yakuza games were all around the 20gb mark for example.

-Just for fun, I looked up the file sizes for FF XIII. 18gb on the 360 and 38gb on the PS3. I'm quite sure that was due in large part to the usage of blu-ray quality cutscenes in the PS3 version.

For seem reason, a ton of games were twice as big on PS3 last gen. Some even had mandatory installs. I think people here said it was because of "redundant data" on the disc or something. 

There won't be pre rendered cutscenes, I think even GTA III was already not using them.

GTA V was one disc only on PS3.

I actually don't remember a single game that used more than one blu-ray at all, last gen or this gen.

Maybe there is, but I can't recall.

Dragon's Dogma Dark Arisen came with 2 discs on some versions for the PS3 but just because they included the actual game without the expansion in a separated disc for some reason (the other had both it and the dlc), then stopped doing it and just packing the single disc both it and the dlc.