GTA V (2013) shifted from the SA three-city template to a one-city-plus-state model. Los Santos plus Blaine County. GTA 6 carries that model forward into Vice City plus Leonida.
Design choices
One major city, dense and detailed. A surrounding state with rural areas, small towns and a mountain. Interiors limited but visible from outside.
The design choice to commit to one big city instead of multiple small ones produced a much more dense urban experience.
Direct influence on GTA 6
Vice City + Leonida is the same design. One major city, surrounding state. The differences are scale (likely larger), geographic style (Florida instead of California) and density (likely higher).