Mad Dog
Another way to look at it is:
Max glide range occurs at the best L/D ratio AoA. This AoA doesn't change with weight, thus the weight of the aircraft doesn't change the glide range.
However, the speed required to achieve this AoA does vary with weight. In the 146 this speed is defined as Vfto (min speed with zero flap) + 30 kts. Vfto also varies with weight, the higher the weight, the higher the value of Vfto.
So if on descent you are say at MLW, you have the highest value of Vfto. If descent is flown at 250 kts, the difference between 250 kts and Vfto is smaller than say if you are several tonnes lighter, with a comensurately lower Vfto.
As a jet's descent is essentially a glide with minimal/partial power, the IAS flown is controlled by (the secondary effect of) pitch attitude.
As weight decreases, Vfto decreases, the split between descent IAS and Vfto increases and, the attitude required to convert the potential energy (of altitude) into the kinetic energy required to maintain decsent IAS must decrease (lower pitch attitude), thus increasing the RoD.