GTA V (2013) moved from the SA three-city template to a one-city-plus-state template. Los Santos + Blaine County. GTA 6 carries that template over into Vice City + Leonida.
Design choices
One large city, dense and full of detail. A state surrounding it with rural areas, small towns and a mountain. Inside spaces limited, but visible from the outside.
Committing to one large city instead of several smaller cities led to a much denser urban experience.
Direct influence on GTA 6
Vice City + Leonida is the same concept. One huge city with a surrounding state. The differences are size (probably bigger), geography (Florida vs California) and density (probably higher).