12mm is the approximate size of the shells OD. All other shotshell sizes however are given by the bore dia of the gun, not the outside of the shell casing.
I have heard, but can't confirm, that 36ga was simply picked as an obvious step down from 32 gauge as the next smaller size was was not a gauge in common use for shotguns.
It is noted that both of the misnomers do tend to make the little pipsqueak appear larger than it really is.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra