These figures are rough, but should help with your query.
At higher weights, the best Lift/Drag Ratio speed is 300 kts.
At lower weights, the best Lift/Drag Ratio speed is 260 kts.
Descent is generally made at around 300 kts, which is great when you are heavy, but not when you're light.
So at light weights and 300 kts (which is the speed for heavier weights), there is a large increase in drag over a 260 kt descent.
The increase in drag brings you down faster.
Now if you understand all that, you doing better than me .