The question was "why does an aircraft descend quicker when it is lighter?" and the answer is that it doesn't - except in the special case where you required to descend at a fixed airspeed above your minimum drag speed. As you all know if you fly at Vimp or Vimd the heavier aircraft will have a higher rate of descent. If you choose a particular descent angle then again the heavier aircraft, going faster, will have a higher rate of descent. If you can manage a terminal velocity vertical dive the heavier aircraft will go straight down faster.
If you are ordered by ATC to descend at a given airspeed or take it to the limit at Vmo/Vne then to hold the required airspeed the heavier aircraft will have to adopt a shallower descent angle and therefore will have a lower rate of descent.
So, in summary, if you are free to choose your airspeed the lighter aircraft will stay up longer. If you must maintain a fixed airspeed in a cruise descent the heavier aircraft will give you the demanded airspeed in a shallower descent
The reason why the question pops up in interviews and in the CQB is because the aircraft behaviour in a cruise descent - well known in practice - appears paradoxical in theory. Anyway, from Keith and Mad Jock you have all the aerodynamics you could wish for to deploy as your answer
Dick