Interesting, i would have expected the SLR to be heavier
It's the same optics, so the RF-version needs a longer body/shell to get the optics as far from the film plane as the SLR version (because the RF-camera itself is). The glass weights the same. The longer body is heavier because it needs more material. The RF-version also probably needs one more helicoid for the RF-cam and that will add some weight too.