Yea I understand the general rationale of cost control, but "5.3 kg" seems a weirdly precise and non-round number given that it's way way lighter than any plausible conventional crankshaft. Was some manufacturer actually at or close to 5.3 kg? That would be truly mind blowing as godlameroso implied.
P.S. yes the new RB has anorexic rear hips, very impressive itself.