I'd think production would be based on the area. Double area = double production.
Double the LINEAR size would be a LOT more than double the production...
It is the AREA that matters in most things like this, but I am not sure WHAT to measure in this case... If it is the area of ground covered(which I would THINK would be the case), then because A = PI * R^2 then 2*R = 4
So if you double the size, you get 4 times the water.