The light aircraft will have MORE drag than the heavy in a, same speed scenario (like in the 767 case above).
The reduced weight will give the light aircraft a shallower AoA moving
it to the left side of the optimum airfoil drag bucket which means increased
drag and having to do a steeper descend to maintain airspeed.
If we do some calculations on the 767 chart, we find that the
Light acf has an average descend angle of -3,03 deg and the heavy acf -2,69 deg.
So the difference is only a 0,34 deg average descend angle.
I'd like to see your calculations..
Are you perhaps ignoring:
a) Heavy a/c has more induced drag
b) The drag bucket only applies to wing profile drag, so isn't the whole aircraft Cdo component (watering bucket effect down)
c) Are you sure where the lighter a/c is on the bucket at ToD? Looks to me (without the chore of checking) it may well be right in the middle of bucket!