A previous dev has suggested that the GTA 6 map must be lesser and much more densely packed. Having said that, as leaks have continued to trickle away from Rockstar, modders have recreated the GTA six map in GTA five's motor which provides a sense of its huge scale. https://gtasixmap43075.ltfblog.com/34283588/an-unbiased-view-of-grand-theft-auto-6