Greg,
I think it's "large" because the mirror clearly must reflect the entire light cone and the mirror has to be positioned at 45 degrees. I think it is about as small as it can be when you add enough thickness to the box to account for rigid tube fittings.
Did you notice that there is also a large model ONAG now? (ONAGXT)
http://www.innovationsforesight.com/...lumination.htm
Peter