San Andreas in 2004 was the big jump. Three cities (Los Santos, San Fierro, Las Venturas), countryside between them, mountains, deserts. The first really ambitious GTA map.
The three-city design
Los Santos = LA. San Fierro = San Francisco. Las Venturas = Las Vegas. Each city smaller than its real-world counterpart, all three connected by a fictional state.
The model influenced every Rockstar open world that came after.
Influence on GTA 5 and beyond
GTA V re-used the Los Santos name and base geography but expanded it. The countryside-between-cities pattern survived as countryside-around-LS in GTA V.
GTA 6 shifts to a single-city-in-a-state design rather than three-cities. Different scale of ambition - one big city instead of three small ones.