Which isn't the question that has been asked.
It is nothing to do with glide distances or speeds in the glide
It is purely
Why does an aircraft descend quicker when it is lighter?
Its purely asking why the rate of decent is higher when you are lighter
Generally aircraft don't operate on the dirty side of the drag curve unless your on approach. Pit Bulls method works for anything above minimum drag. I know these days of advanced fuel performance the wisdom dictates we don't decend at Vmo but its always at a speed which is faster than minimum drag.