My best guess, as a former glider pilot and a few hours in helicopters, that it has to do with the speed.
In gliders there is an optimum glide ratio at a certain angle of attack. Max CL/CD ratio.
The heavier the glider the higher the speed at which this optimum is found.
So a more lightweight equal glider will sink faster than the heavy glider when flying at the speed at which the heavy glider is at max CL/CD ratio.
For helicopter in autorotations there are also speeds for optimum glide ratio and minimum sink rate.
Tim