Is it the fact that it is stiffer and lighter than smaller modulous rods, or is it due to the composition of the higher modulus graphite fibers itself? Or both? Or am I missing something?

It's a matter of stiffness to weight ratio. For any given stiffness, the lighter rod of the two will be more sensitive. For any given weight, the stiffer of the two will be more sensitive.

Which then leads to the next thing - the material itself is not the final word on the matter. The structure itself is also involved. There are different ways to use fibers - thin wall, large diameter, thick wall, small diameter, etc., etc. So a rod made from a higher modulus fiber can be more sensitive, but depending on how it's used it may not be.

The higher the stiffness to weight ratio (ERN to physical weight) the more sensitive the blank will be, regardless of the material used to make it.

.................

Awesome. Thanks, Tom.