I have noted the following experience:

I can grind the same quantity (mass) of the same beans into the same portafilter using two grinders with approximately 25% difference in volume.

The two grinders are a eureka Olympus wth red speed burrs and a mazzer mini with super jolly burrs. The mazzer produces the greater volume.

What about the grinders leads to such a difference?

The coarser the grind, the greater the volume. That's after tamping.