#9 is just a tiny bit smaller than 71/2, 8 or 81/2. With the naked eye you can not really tell them apart. From 71/2 - 9 there is only a .38mm difference.
7 1/2 - .100" (2.41mm)
8 ------ .090" (.2.29mm)
8 1/2 - .085" (2.16mm)
9 ------ .080" (2.03mm).
