I can't say how big that deers neck is but my neck mesures 18.5 inches around the smallest part and that deers neck is a heck of a lot larger than mine.

Judging fromcomparing the deers neckand your neckin the picI would have to say that 21 seemsa bitsmall. Even taking into consideration that you are sitting behind the deer.

In measuring a few mounts in the shop that are Florida and Alabama deer they measure 19 - 23 inches in diameter. These are 150 - 180 pound deer.