Right, I was ahead of myself and counted the number of atoms wrong.
It sounded counter-intuitive to see a smaller fluctuation with the
smaller pdamp parameter - usually it's the opposite. Below is taken
from the doc page
"IMPORTANT NOTE: A Nose-Hoover barostat will not work well for
arbitrary values of Pdamp. If Pdamp is too small, the pressure and
volume can fluctuate wildly; if it is too large, the pressure will
take a very long time to equilibrate. A good choice for many models is
a Pdamp of around 1000 timesteps."
Did you use the default timestep? If yes, can you try with a smaller one?