Guys lets keep it simple....lets not get into L/D ratios and coefficients of lift....it has absolutely nothing to do with that.
The answer is : a heavier aircraft needs to descend earlier because as any aircraft is restricted to a certain VMO/MMO for descent, a heavier aircraft has a higher momentum and so needs to start descent earlier and maintain a shallower rate of descent or else it will overspeed as a result of the weight driven momentum.
The shallower the rate of descent the greater the ground speed. You cover more ground distance per every 100 feet vertically being heavier. So simply, a heavier aircraft needs to start descent earlier than a lighter aircraft, as a lighter aircraft still being restricted by VMO/MMO for descent, is less likely to overspeed as a result of its weight driven momentum.
Hope this helps
cheers