Well in theory there are more PPs given out for bigger games - because there are more players.
However, a small multiplyer - like 10 for 1v1, 12 for 2v2, 14 for 3v3 and 16 for 4v4 would probably see a lot more of these games being played.
All this is moot however, as the game mechanics will be changing when the warmap goes Beta ;p
Larger games will give bigger (better) warmap rewards however