Whilst I'm happy to run with up to six players (though I don't think I could handle more than that), I prefer to run with small groups. 1 GM and two players is ideal for me, provided that you can be sure of their commitment. But then in my experience, there is usually a core group of committed players in any group, and it's the same less-than-committed players who pull out regularly, the players who make up the numbers. Given that as a GM I go out of my way to try and include every character and give them subplots to run with, a no-show from any player is equally damaging. I'd rather stick with a small group of players I know are as keen to play as I am.
So there's that consideration, and then I just find that two players makes for a really dynamic team, that actually gets things done rather than argue about convoluted plans, and works well together. There's no fifth wheel or noisy fighter to alert the guards, and the two PCs need each other, making for a more tense, exciting and fast-moving game.
I've never tried a one-on-one game though, and I'd be interested to give that a shot some day. I suspect it'd have its advantages, in terms of fitting better with traditional narratives, but that the disadvantages would probably outweigh things in the end.