Girls comparing cock with their underarm

Never had a gf do this to me in actuality, but considering that my shaft is almost as thick as my wrist is (like 85% of my wrist's thickness), I'm gonna assume my shaft is thicker than most of my exes wrists, or at least approximately the same thickness.
 
Isn't the average size of a woman's underarm, 9.5"?

Every time I see someone do this with Mandingo, it's pretty much bob on the same length, which would put Mandingo around that size.