According to the reviews that mention it, the game does open up after the prologue and proper starting area, I'm only about half an hour past the prologue and still in the first area so I can't say for myself how much that's the case.
Will say the prologue got me nervous as fuck though, it gives you some assault rifles and you can find decent amount of ammo right off the bat, but at least by the end of it you're reduced to only your crappy pistol with little ammo and a detector when you're thrown into the proper game.