Both are 20mm in diameter.
It's just shorter.
With a mk22 (original mk22 not the new one that came out last year) cap on the m221b it is about the same length as the m222/cmcX body.
I think the length of the innards (circuits) are the same size (i.e. the tube around the components) and the difference is XLR vs the Tunchel connector.