Pack is based on the speed of the shock oil passing through the holes in the pistons. A smaller amount of hole area will lead to more pack and a larger amount of area will lead to less. There are some places online that get this backwards.
It is common to hear talk about the surface area of the piston being greater, which is the same as the surface area of the holes being smaller. Just thought I point that out since after reading through most of this thread, I have seen it referred to both ways, which both are correct.