Yeah, it doesn't seem like this was written by someone whose native language is English. It seems to me as though Small will use the least CPU for this portion of the job, and Average will use a medium amount of CPU, and Large will use the most amount of CPU. And I believe it is talking about how much CPU to use in the area where it matters the least, at the coarse end of the hierarchy. Once it gets to the fine end of the hierarchy that is where spending more CPU will make more of a difference, or show better results. I think what it means is that at this point of the job the amount of CPU that you spend will not be as important so you might not want to bother spending more CPU since it should affect the final result the least. You can set it to Average or Large if you still want it to spend more CPU here, but the effect on the final result may be what is called diminishing returns. You won't likely see a difference compared to the amount of CPU spent.
I could be wrong.