Hm, that first shot from the Observatory looking down at the city finally gives us a good look at what's down there in the south... a bridge to another island? In fact the trailer shows several shots of bridges, maybe all that same one. It looks interesting over there.
So Los Santos may be a little bigger than people have estimated. Might actually be as big as Liberty City.
Actually I believe in real life that bridge is located near the Long Beach area. So I doubt it leads to another island.
And they have said Los Santos alone will be a few times bigger than Liberty City. We haven't even seen close to how big the city portion of the game will be. How else do you expect the entire map to be bigger than GTA IV, San Andreas, and RDR combined with a Los Santos that's only just as big as Liberty City?