As @LivingnLA said, it has to do with anatomical/physical limits... For years I thought only a finger will fit back there. Then, little by little I started exploring two fingers, three fingres, 7's, 8's, and 9's of different widths. At 10" I stopped because I believe I have reached my limit of tolerance/pleasure. I first experimented with a skinny 10 and saw that I could handle it, so I tried other's of different width and that's where I am at... I certainly am not the type that would venture into FF. Cheers!