Alleged GTA 6 Map Size Vs GTA 5 Looks Amazing And Fan Hype Explodes
Ultimately it comes down to the size of the map. While GTA V's map is quite large compared to a lot of other open world games, it also has a modern setting with lots of fast vehicles, including supercars, hypercars, and even things like jet aircraft. Realistically, some of these vehicles should be able to cross this map in a matter of seconds, as it's only a few miles wide.
This is probably why the game implements a radically reduced scale for vehicle speed. Most vehicles move at a maximum of half their stated speed, so a car that says it's doing 140 MPH is actually doing something more like 70 MPH. Aircraft, particularly jets, move at even further reduced speeds for gameplay purposes. This is fair enough, but it does detract from the realism of the game, and it makes driving the super-fast cars you can earn in the online mode somewhat less exciting.
Well, that may not be the case in GTA 6. As you may recall, there was a big leak last year of pre-production content for GTA 6, and that included quite a few videos of an early version of the game with a developer overlay enabled. One of the things that fans were able to see in those videos were the coordinates of the player character. GTA 6 is likely to use an evolved version of the RAGE engine that Rockstar has been using since GTA IV, and fans are quite familiar with the coordinate system that that engine uses.

By plotting the coordinates shown in all of the leaked videos, fans were able to put together a map that gives us a rough idea of how big the playable area in GTA 6 will actually be. The short answer is, really frickin' big. Overlaying the GTA V map over the GTA 6 map and matching the scale using the known RAGE engine units, it looks like GTA 6 could be at least twice as large as GTA V, if not even bigger.
The map above was put together by a YouTuber, but the map below is the result of a community effort to try and start creating an early map for the game based both on leak content and also on extrapolation from that content. Interestingly, if this map is correct, the {0,0} location of the GTA 6 map leaves a lot of room to the south and east, possibly allowing for islands that weren't present in the leaked content. Since GTA 6 is known to take place in Miami analog Vice City, it's possible that we could venture to a Cuba or Haiti analog.

It's also possible that the map extends to the north. We know from earlier leaks that in Rockstar's original plans for the next GTA game, the company wanted to have a much larger map, including multiple cities the size of GTA V's Los Santos. It's easy to imagine the map extending to the north and including representations of Northern Florida cities, like Orlando and its famous theme park, or even possibly analogs of places in Georgia.
While this information is based on logical deduction from actual verified leaks, it's also possible that Rockstar's plans could have completely changed since that information was leaked. The large distance between coordinates of the locations in the leaked videos could simply represent the fact that these locations were spread far apart to allow for simultaneous development without interfering with other areas of the map.