See, it's obvious that the nanosuit is one of the main points in the crysis series, and in crysis 2. So they need to make the nanosuit feel powerful. How is it going to if your overwhelmed by hordes of people all the time. Scale the maps bigger? That ruins the whole effect of how powerful you feel in the game. In a 6v6 your less likely to get your neck snapped in the back by another enemy while facing off against his teammate.
If there were ALOT more people, well, judging how c2's showcased maps look, i'd say you wouldn't even have enough seconds to live to get a kill on one person.
The only real problem with 6v6 is the impact on the community, with clan battles and such...
However I really like the idea of playing different modes with different amounts of people on it. Really hope they incorporate that in the game.