It is SMALLER (not by much) not bigger, yet as explained in one of the other thread regarding the SlimLine, it has a bit better gain and better engineered design.
The "SlimLine" is 32.25" x 22.50" = 725.625 sq in surface area as compared to 29.50" x 25.50" = 752.25 sq in surface area for the AT9. (SL is 26.625 sq in smaller)