If the game world is like GTA5, then that install size will reflect tons of unique items, like buildings etc which are used in one area only and not instanced all over the map. Most games will instance buildings, but I think GTA5 has very few instanced buildings.
GTA V has a lot of objects cut from the game still there, pine trees, a bunch of animation for cats not seen in the game, i guess the rumor of buying pet was real, cos they are all domestic animations, and there is a pet store in the game,lots of buildings and 3d assets, and unique animation peds dont use
