What's the average field for all the PO games? Anything above that should yield a bonus of some percent, maybe like 20% of the how much bigger it is maybe.. aka an average field for all the tournaments is lets say 40 people and a tournament has 50 people, that would be 2 bonus points (20% of 10 person difference).
This might have to be the average of that actual day tho, otherwise as the fields grow, this system won't hold long term weight.