If the interview board accept an answer that is concerned only with the fact that the heavier aircraft has more energy, this will relieve the candidate of the burden of demonstrating even the most rudimentary knowledge of aerodynamics.
This is exactly the wrong way to eliminate the large number of recent/current JAR ATPL students who pass their exams by simply memorising answers from online databases. Mony of these candidates have only the most rudimentary konwledge, so a little bit of probing will quickly separate the wheat from the chaff.
The only way in which the technical interview can serve any useful purpose is if the board ask questions which cannot be memorised prior to the event. This will of course require more effort on the part of the board.
If the slower rate of descent of the heavier aircraft were due entirely to its greater stored energy, then this would be true at all airspeeds. But it is not true at all airspeeds. Given the right combination of weights and airspeeds, the lighter aircraft will sink more slowly.
The relationships between the two weights, the speed chosen and the relationship between that speed and the Vmp for each aircraft all play a major part in determining the rates of descent.
If the board wish to make the process worthwhile I suggest they use a series of questions such as:
a. What effect does increasing aircraft weight have on maximum glide range? Answer none.
b. What effect does increasing aircraft weight have on maximum glide endurance? Answer increasing aircraft weight decreases maximum glide endurance.
c. So why does my 777 require more time and distance to descend from cruise level when it is heavy than when it is light?........
This series of questions might also shed some light on how the candidate will deal with unexpected emergencies.