Ground size for 100 amp sub panel 200ft away
You start with an 8AWG copper wire...

You are correct that for a 100A feeder, the associated ground wire needs to be an 8AWG copper or 6AWG aluminum wire.  If you are using a four-wire direct burial cable, then just use the ground wire in the cable as it will be quite amply sized for the job it is doing; if you are using PVC conduit, then bare 8AWG copper would be my choice to keep conduit fill down.

...and then upsize from there based on how much upsizing of the other wires you do

When you start upsizing wires for voltage drop control, the size of the ground wire is governed by NEC 250.122(B):


  (B) Increased in Size. Where ungrounded conductors are
  increased in size from the minimum size that has sufficient
  ampacity for the intended installation, wire-type equipment
  grounding conductors, where installed, shall be increased in
  size proportionately, according to the circular mil area of the
  ungrounded conductors.


For instance, using 1/0 aluminum wire for the hots and neutral in your example vs. the 1AWG that is required to meet Code ampacity rules gives us a size ratio of 53.49mm2/42.41mm2 based on the NEC Chapter 9, Table 8 conductor sizes (we can do this math in mm2 since the conversion factors cancel out here) or about 1.26.  Multiplying that by the 8.367mm2 area of an 8AWG copper conductor gives us about 10.55mm2 for the resulting wire.  This isn't a standard conductor size though, so we should use a 6AWG copper conductor (which has 13.30mm2 of area) or a 4AWG aluminum conductor for this ground wire.