Mike, I think you are right about 8x10 being no different in theory. In practise, such things as big feet are really obvious on that big glass!
I am going to try the shot again after work today in two different ways. 1) I am going to get back a little further and expose a sheet of 8x10 with frame not quite so full, and 2) from the same position, I am going to use my 5x7 back to fill the frame and shoot a few that way.
I am just guessing that you think working distance is very important.