A light airplane can descend more steeply than a heavy airplane, but it doesn't have anything directly to do with momentum or acceleration.
Given the same airplane at 2 different weights, the speed for maximum glide will be lower for the light airplane, because the angle of attack for max glide will result in a lower airspeed. Remember that the glide ANGLE at max glide is the same, regardless of weight.
Take, for example, a B747. At max takeoff weight the max glide speed is around 260 KIAS; at light weight it can be as low as 210 KIAS. So, the lightweight 747 descending at 260 KIAS is descending much more steeply that the heavyweight, because it is flying 50 KT faster than optimum.