If that is bigger than the Afco, it is not by much. It's not a matter of not wanting to make it bigger, its a matter of where to put it.
I would wager that the two could be compared and a measurable difference in performance could not be found.
Not sure of the difference in diminsions.. It uses 11" fans instead of the 10", so I assume at least 1 more inch of surface area all around? Would be nice to see some data sheets on it compared to the basic c&r and afco