Use the upside down drag curve graph

At a given mass best glide angle is at Vmd and the corresponding IAS for that weight.


If we now lower the mass but keep the speed for the original mass we will not be at the best angle of attack for best range in the glide, the descent angle is steeper.
ROD = SINTHETA x TAS


Theta the glide angle is steeper and the effective IAS/TAS is higher than the new IAS/TAS at the lower weight, therefore ROD increases.