It's a pretty long stretch from THE biggest. JC2's map is 386sqMi or 1000SqKM. For a direct contrast, FUEL's map (An open-world racing game) is over 5000sqMI or 14,000sqKM, IIRC. There's a couple RPGs with a bigger map than that (I forgot the name of it), and I think even Mercenaries 1/2 had a bigger map than Just Cause 2.
However, it IS up there for the larger game worlds, just not the biggest.
technically there are games that comprise multiple worlds..
star wars galaxies has somewhere between 6 and 10 full worlds to explore, fight and build on... granted it's shared with other people. but you can actually drive across each world in probably about 1 hour each.. not sure it's been years since i played and didn't really test it even then, just guessing.
on the other hand everquest has a HUGE planet, water areas, a moon, underground areas of both the main planet and the moon and alternate planes of existence, plus instanced missions of the past world. plus just the main continent on that world has several zones and would take you a couple hours to run across from one side to another, which would be like running straight across the middle of the U.S. you'd still miss a lot of the continent.
so technically the sq footage of everquest is a lot smaller, but the area would take a lot longer to travel.
i've heard that eve online is even larger..
RTS games also allow you to conquer a planet, or multiple planets.. even an entire solar system.. turn base strategy games like the greatest ever "stars!" you conquer a galaxy.
spore features a whole galaxy to play in, starting from a tidal pool and evolving up to space travel...
but as for similar games to this... i'm actually curious how it'd compare to Prototype.. which is a fun game, but it could be bigger area (it's a single city) and more replay value would be nice. i haven't tried just cause 2, but it's starting to look interesting.
why in the world would i capitalize the word i? it's NOT a proper noun.. we don't capitalize me.. most importantly a capital i looks like an L for some reason.