OK, we are talking about same model at the same descent IAS? Like a heavy "full house" A330 and an empty A330 flying at 300 kt?
Then it is all about L/D ratio.
The distance for altitude is equal to the L/D ratio.
L/D ratio depends on AoA. L/D ratio increases with AoA up to a maximum (min Drag AoA) and then decreases again. Normally we fly at AoAs below max L/D AoA.
Imagine those 330s. The heavy one, flying at max L/D (green dot speed) would make, say 240 kt, whereas the light one would be making only 210 kt. But they are both flying at 300 kt, so the heavy one is 30 kt closer to green dot that the light one. Its L/D ratio is better. Its AoA is closer to min Drag, to green dot. That is why the heavier airplane flies more miles for the same altitude.
Another way to look at it. Imagine two blocks on skis on two icy downslopes, sliding at 120 kmh. The heavy one will have enough forward force to overcome the drag of those 120 kmh, with a shallower slope than the light one, due to its weight.
As for the second one: speed increase is the same as reducing AoA, which means getting farther from min drag AoA, and decreasing L/D.
Last edited by Microburst2002; 21st Nov 2012 at 19:12.