Why does an aircraft descend quicker when it is lighter?

[jimmygill]

Glider Performance Airspeeds

I hope this picture is sufficient.

Few things to notice.

1. The heavy glider and the light glider have the same best L/D ratio, note the two polars share the same tangent passing through (0,0).
2. The angle between the tangent and the horizontal axis is same as glide path angle.
3. If you fly both the gliders for maximum range, the heavier one will have higher TAS and higher sink rate, but will have same glide path angle as the lighter one.
4. If you chose to descend at 70 Knots TAS, the heavier one will descend slower and shallower than the lighter one.
5. If you chose to descend at 48 Knots TAS, the heavier one and the lighter one will both descend at same sink rate and hence same flight path angle.
6. If you chose to descend at 40 Knots TAS the heavier one will descend faster and steeper than the slower one.

@mad_jock
Please note that picture looks very sexy to me, but the picture is an observation not an explanation. Can you explain the above picture with your weight energy hypothesis?