@ Daverz, you are correct in linking that recommendation with the room size.
Dunlavy made seating recommendations based on a list of priorities.
The first was to always use the long wall if possible. This was to first increase the space between speakers but, just as importantly, to increase the relative distance of the direct sound versus the first reflected sound from the side walls.
The second priority was to keep the speakers off the wall behind them as much as possible. This also was to keep the difference in timing between direct sound and reflected sound relatively high. It also helps to increase sound depth.
Of course, all Dunlavy speakers were designed for a 10 ft. listening distance. So the distance from the listener to the wall behind him/her was the last priority. And in normal size listening rooms usually meant the listener was right on the wall. In this case, the easy fix is to cover the area with thick absorptive material. Leaving this wall for the primary wall treatment also meant that you could get away with a lot less material since the closer you sit to the wall the smaller the area of treatment necessary.
Given these priorities it is easy to see how the size of the room will ultimately impact the placement recommendations. Hope that helps!