maybe I had the same problem ( with SAI ).
What I called my "a screen inside a screen" problem.
The mouse uses the whole screen but a brush only a much smaller part
of the screen. Let's say a 30% shrink.
Maybe it's a tablet preference issue in Gimp if you have no problems
with other graphic programs.
A recent driver update to blame?
I have replaced my graphic card.
But before replacing (all) your hardware give the tablet a try on
another computer and see if you still have the same problem.
I never used your pen, so I don't know if that could be part of the
problem.