I think it's because bucks have something that's comparable (antlers). One can readily measure the antlers for comparrison to their buddies buck.

Well, I'm thinking of starting a new trend....... The Big Doe Club. For comparrison, we'll measure their teats! What do you think.......length and circumference for all four?

T-1l
T-2l
T-3l
T-4l

T-1c
T-2c
T-3c
T-4c

Total teatage............. just gross, no net! [:-]