Just for comparison sake, here's a pic of the BW .88AR undivided T4 housing (left) compared to a T67 .68AR T4 housing (right).
The AR numbers aren't really that much different, but remember AR is just a ratio of Area to Radius, so the numbers can be a little deceiving. The pic makes it pretty clear how much larger the housings are and how much more air they flow. The "R" is obviouslly way bigger, so the "A" must be much larger too.