Not in terms of literal game size, at least. No, the honour of the game with the biggest world will surely go to Bohemia's upcoming ArmA III, which takes place on an island so damn big one of its designers went out and made this comparison chart.
Drawn (roughly) to scale, the top half are maps from previous games/expansions in the ArmA franchise. The big map in the middle? That's Altis, from ArmA III, which is even bigger than it looks because you'll be able to explore much of the ocean floor around it as well.
The maps down the bottom are worlds from games like Skyrim, Red Dead and Grand Theft Auto. Oh, and Counter-Strike, just for laughs.
You can download hi-res versions at Karel Moricky's site below.