I actually handled the Remington R51 today at a gun shop. Visually, it seemed OK. Physically I'd say it was about Makarov size.
Cycling it by hand, oh, boy! Definite hitch halfway through the process. Probably where that free floating breech block thing disconnects. I hope this gets better with wear in and appropriate lubing.
Give it a couple years in the field. I'll stick with my XDS.