The 645 is normally smaller than that, unless you carry it around with the 100mm. Seems odd to compare the 645 with a lens nearly twice the normal-for-format focal length with the RB and a wide angle that's closer to (just over) half normal-for-focal length. But I guess that's what you had. The RB is still 2.5" taller and a whopping 2.25" longer with a wide angle lens than the M645 is with a moderate tele.
I never owned one but from looking at them the RB always looked to me like it would be similar to carrying a very light weight cinder block around.Light that is as cinder blocks go.
I was just measuring what was sitting around, the way it was sitting around.