I never bothered to notice, plus I usually only do 1v1 rooms; but more people in a room means more exp/gp/skexp per win/loss iirc.
Don't chew my head off if I'm wrong, I honestly never think about the exp I'm getting when I pvp. .-.
I don't believe it does. All my PvPing is done in either 1v1 games with my my couple or in 3v3 rooms with my couple, and I've never noticed a difference in my exp gains.