I am not a big fan of the G21 grip either. Too big, and too square and blocky. I got rid of mine but still have a G20 which is the same size.Well I hate the G21 and it’s been a number of years since I’ve handled one. So I am no help there.
As for the grip. Side to side MK23 and USP they are virtually the same. At the widest spot of both the MK23 is 3mm bigger forward and aft