Even in that context GTA5 seems to have a lot more though, a massive city with all types of places like deserts and downtown areas, planes, cars, motorcycles, and so on.

Yeah, visual assets up to very high resolutions is a large part of it. I wish I remembered the game, but I do recall one (it was a free to play battle royal game, not fortnite or PUBG) that had an ~15GB base game download, and then an addition 30GB for 4K textures.

In the case of GTA I’d imagine the voice lines are a major source of size as well. They’ve been doing updates for the online with new missions and voice lines for ages.