If you take the zerk coupler apart you will see there is a rubber piston that pushes on and closes the jaws around the zerk head. To release the zerk coupler you have to push on it axially to move the piston back and release the jaws. Then pull the coupler from the zerk. If the zerk is plugged or the pin is too tight (the coupler remains pressurized) it will take quite high force to push the piston back and release the jaws.
I don't think it is that. I think the are larger. It feels like you have to push it on the fitting harder too. I guess I should measure them to see how big they are.