I don't know much about the XAircraft MR offerings but the general rule is as follows:
Smaller MRs with shorter motor arms tend to use higher kv motors. Whereas the longer motor arm lengths that allow for larger prop diameters use lower KV motors since lower KV is much more efficient especially when turning larger diameter props.
My Cinestar 8 has 1100 mm motor arms that allow for 15 x 5.5 props. That in turn requires me to require motors that offer much higher torque (with a tradeoff on the motor speed). So I am using Tiger-4014-11 motors which are 400 KV.