The root volumes for our three vservers are all only 0% or 1% used (1GB each).

But the "vol0" volumes show:

scarab::> vol show
Vserver   Volume       Aggregate    State      Type       Size  Available Used%
--------- ------------ ------------ ---------- ---- ---------- ---------- -----
scarab-a  vol0         aggr0_scarab_01 online  RW      151.3GB    78.82GB   45%
scarab-b  vol0         aggr0_scarab_02 online  RW      124.6GB    61.74GB   47%

I think I recall trying to resize a node's root volume from the CLI in the past and getting an error along the lines of "don't mess with this volume."