The problem with JC2 is a lot of it is jungle or water with nothing much to it (although occasionally there's stuff on or under the water).
It's like saying "WOW! SR2 has this MASSIVE map!" without noting that a lot of it is empty ocean. Sure it is a big map, but should be honest about it.
Then again, that is the tendency of R*.. they take a game from before, slice up a part of the map, and make it bigger and packed in (GTA4 is only part of GTA3's map but sized up, and now GTA5 looks like the first island of GTA:SA but sized up)
It's not all bad.. the big question is how much stuff is put in it. That's the problem with the games.. it's not that they have tiny maps with nowhere to go (that's the opposite, they have great big maps to play in full of stuff), it's the gameplay. Even a huge map never kept me engrossed in GTA4.