Largest game worlds I believe would be The Elder Scrolls 2: Daggerfall. The map was around 487,000 square kilometers. There were roughly 15,000 locations to visit in the game and roughly 750,000 NPC's.
Finished that Outcast Chinese Sim game... then some of the outcasts turned on me! I had to fight them off. That was a sweet weapons score though. I am finally at the point in the game where I can use 100% power weapons and not worry about Money. I have over 1400 caps.
I think I found another DLC spot. The Car Tunnel at the bottom right hand porition of the map. That poor robot got destroyed.
Passed 60 hours total play time to reach #7 on my all time most played PC game.