Greeetings Gents,
It is my understanding that for a given speed in descent the heavier a/c will descend at a higher rate. Conversely for a given rate of descent the heavier a/c will descend at a faster speed. As top of descent point is based on ground speed the heavier you are the faster you'll go hence the earlier you must start to descend.
This is my take on that. So it's only worth the paper that it's printed on. Hope it helps though.