Olendirk,
I agree with gearpin and Scientist versions but in other words my answer is:
Because an aircraft is restricted to maximum speed during descent, the heavier aircraft has to maintain a lower rate of descent than a lighter one, otherwise it would overspeed. Remember, heavier aircraft have a greater momentum and this weight driven momentum will produce a greater speed in in a vertical dive. Therefore, a heavier aircraft has to start its descent earlier than a lighter aircraft because it has to maintain a shallower descent.
In other words, a lighter aircraft can descent later and quicker than a heavier aircraft because it can maintain a greater vertical descent profile without overspeeding.
Hope that answers yours question.
TFCM
