Checkboard,
I think that you will find that this is because "line flying speeds" are close to Vmd and therefore higher than Vmp for both the light and heavy aircraft. This means that if they are both flying at such speeds, the heavy aircraft will be closer to its best glide endurance speed (its own Vmp) than the lighter aircraft will. So at such speeds the light aircraft will have the lesser endurance and hence higher ROD.
But if you were to glide both aircraft at the Vmp for the light aircraft it would (probably) have the greatest endurance.
FT,
Your equations are interesting but I am not convinced. If I may quote you,
"Then the question boils down to if the drag at 30 tons is less than 57% of the drag at 52 tons at the given airspeed. If that is true, the heavy aircraft will descend faster....I find it highly unlikely".
Well Cdi is proportinal to Cl squared and at a common speed the Cl at 52 tons will be 52/30 = 1.733 times that at 30 tons. So the Cdi at 52 tons will be 1.733 squared = 3.00 times sthat at 30 tons. This means that the induced drag at 53 tons will 200% greater than that at 52 tons. or to put it another way the induced drag at 30 tons is only 33% of that at 52 tons.