Because every ounce of weight is extra oxygen demand. Extra Lactic to hinder body, extra load on the bones both inside and out, higher uric potential...
He has very big arms and upper body now, this is silly for an elite (potentially) athlete of small build, IMO. 10K is one thing, but this is short burst high runs then jogs, hard changes of direction.
If you build the picture out, it's clear the way is going (tackling less and less prevalent = game has much longer flows between breaks), muscle is soo last decade.