How many posts!
It is all about L/D ratio. Of course we can look at it from the point of view of Energy or Momentum if we take into account all the factors.
However, the L/D is the best explanation.
1-The gliding ratio is equal to the L/D ratio (By geometry)
2-The L/D ratio is equal to de CL/CD ratio (Dividing L formula by D formula)
3-Both CL and CD depend on the AoA alone (and Mach Number, ok...)
3-For a constant IAS and straight flight path, the heavier the airplane, the greater the angle of attack (From Lift formula)
4-flying in the stable speed region (AoAs above best finesse) the heavier has a more efficient AoA. (see any CL/CD versus AoA graph)
5-Therefore, the heavier airplane has a better L/D. It glides more miles for altitude.
If the descent speed was very very low so that the heavier was flying in the reverse command region, wich would be nonsense, it would glide less than the lighter one, because it would have a less efficient AoA.