Alright, lets consider it this way. This will be easier to comprehend.
Are you aware of the forces involved in a descent. They are Lift, Weight, Thrust, drag and forward component of weight.
Increasing the weight, increases the forward component of weight. Hence the ROD increases and vice versa. If the ROD increases, the forward speed increases. While descent, there is a limit speed in most cases. So if we have to keep the speed under a certain limit, we have to start at an earlier point so that we can descent at a lesser ROD.
Is that helpful ? If you're finding it difficult to understand the forces in a descent, message me. I'll email you a presentation I have for descent. It graphically explains you the forces and it will be much easier to understand.
Regards