I have to disagree here.
The only way heat is transfered from the coolant to the air is through the fins.
The only way the heat is transfered from the coolant to the fins is through the tubes
The only way heat is transfered from the coolant to the tubes is by contact to the wall of the tubes.
The more surface area of the tubes the more heat that can be transfered.
The more contact points / surface area between the fins and the tubes the more heat can be transfered.
So if a 1 row core can transfer 100 btu in a set time then 2 cores can do about double, 3 cores can do about triple.
The reason I say about is there will be warmer air hitting the middle or last row of fins / tubes making them a little less efficient but they will still be effective.
Also...
If you go from 10 fins per inch to 20 fins per inch you will have about twice as much heat that can be transfered.
Of course there is a point of deminishing return where the fins per inch start to block airflow but that is no where near where radiators are.
