I've experimented with estate size and efficiency fairly extensively, and I still haven't reached any conclusive conclusions. In one city, I folded all empty estates into my margrave estate and only saw the tax return for that estate increase by 8 gold. So it seems like the lowering in the efficiency because of the large percentage size must have had some effect, since the empty estates were making more than 8 gold between them before they were folded into the larger estate.
