The reason your 747 (or any other aircraft in this scenario) is coming down at a steeper angle at the lighter weights is a bit of a different issue than this thread. To put it simply, you are descending on a constant speed schedule that is not adjusted for weight. However, your IAS/Mach for L/Dmax _does_ vary with weight. The typical transport descent schedule speeds are well over the L/Dmax for any weight. So, at the heavier weights the corrosponding higher L/Dmax for that weight happens to be closer to your actual speed, which nets more glidepath.
Can also do this with vectors, but then I'd have to somehow draw pictures here.