I hate to say it, but I think Killzone 2 will be huge-er. It's a shooter. With good graphics. Like Crysis, Call of Duty 4, and Gears of War.
As sad as it may be, it will sell pretty well for these reasons alone, and when you add in the fantastic gameplay, it'll probably sell well over 4 million, with a high chance of eclipsing LittleBigPlanet's lifetime sales.