Game size are not determine by the size of the map most of the time but the amount of assets you kept inside the shipping build. Usually the size of the files ranked are textures, audios(especially if you support multiple language), cinematic (pre-rendered), animation.
edit: MooseBoys reminds me how much cosmetics we have now in our games.
edit2: If game engine allows artist to paint over game world and save painted virtual textures tiles for location decoration purpose, texture will scale with game map size, see my response below using BG3 as example.
Will be interesting what GTA6 will bring to the table. The visual fidelity will probably surpass and be as big as RDR2.
If GTA6 doesnt surpass MW3 I feel like it has no place to be bigger. Even if MW3 supports multi-dub, cinematics, etc.
Actually, my statement might be a bit wrong regarding map size once I think more carefully. Modern trend that allows artist to directly paint on game world could create really heavy virtual texture assets that scale with size of the world. Games that approach “unique” look or feeling per area without making you feel they reuse or have tileable textures all over the place tends to use this as you can just stream in textures that mask over tileable and make it looks really decorated for that area. They basically trade file size with artist freedom.
One example is BG3, where the VirtualTextures_*.pak have 18 files, 72GB. While normal asset textures has only 4 pak files and aobut 13GB.