I certainly don't claim any kind of expertise in this field but the first thing that springs to my mind is airflow limitation through the cowls.
If you have multiple rows of cylinders the cooling air which each row receives is going to be hotter than the preceding row, due to the air having already passed around a row, or more of cylinders. Also if the rows of cylinders are off set then the airflow is going to be slowed more than if it is passing over only one row.
The amount of airflow will be dependant on the diameter of the cowl opening which will of course be limited by drag considerations.
As I say these are just my gut reactions and may well be way off the mark. I'm sure many people, more knowledgeable than I, will be along presently to give detailed responses.