The most simple approach to this question is to note that the trangent to the drag curve touches at Vmrc. This is typically in the region of 1.32 Vmd. For jet aircraft this is the maximum still air range speed Vmrc.
As weight gradually decreases due to fuel burn, the value of Vmd gradually decreases. This causes the value of Vmrc to decrease.
The figure 1.32 is not absolutely accurate and for any given aircraft type will depend on the shape of the drag curve (deep and narrow or shallow and wide). 1.32 Vmd is simply the "conventional wisdom" figure for JAR exams.