This all unsurprising for a minimum inverter and any kind
of line length. You could probably find fF/um or fF/um2
numbers in the PDK docs if you want to sanity-check the
extracted result.
Check that you have not simply used improper layer/purpose
for your polygons/paths - like, use Met1/drawing, not
Met1/net or whatever. Again you could go to the PDK docs
or the layertable to see what datatype means, in the kit.
The nplug/pplug stuff, I'd guess means a wrong choice about
how to do body ties / taps - or that the actual connection
needs something more (like, maybe "pplug" is a "recognition
layer" that should mark a pplus contact to sub!).