I'm using this as anecdotal evidance, not a rule of thumb. It is not meaningless.
It is just to show that the loads we put on the rigging line are not just the weight of the load we are lowering. Weight is just one force we are dealing with. If we are going to advocate dealing with bigger wood, we need to advocate understanding the forces involved. Not total comprehention, because there are too many variables involved. Add to Mike's list the experiance of the PW operator. Any friction in the tree...
We have people argue that the force multiplyer on a rigging sling will not be X because of friction other places. So what, there is significant additional force in this part of the system. Be aware of it.
If you are going to run out big pieces with a PW be aware that the friction breaking will add siginficant force to the load being run.
I can stop a load dead on my port-a-wrap
Which is why I always try to state that Dave had the load run properly, gradual braking.
With the proper equations you can figure what force is needed to stop load X from falling Y feet. Sice speed tripples with each unit of measure, it is obviouse that you would want the line that is catching the load to be as short as possible.
Which brings me to
but I am having trouble seeing how the PW can multiply the load on the rigging line
Disapating kinetic energy, speed, velosity.
Your still seeing the load as a 500# chunk of wood. See it as load X being chunked out from Y feet. As Y increases then the speed does also. so the force that the ground needs to absorb is greater, hence bigger holes to fill in.
Or to be a little more direct, the same chunk dropped from a taller spar makes a bigger divot because it has more energy in it.