Roger, while there does appear to be confusion between a GEC vice EGC, I have to disagree with your statement regarding not upsizing the GEC.
Table 250.66 "Ground Electrode Conductor for Alternating-Current Systems" does not give any exceptions for oversized conductors, nor state the size in terms of current, but specifically requires GECs based on conductor size.
Even when a "Grounded Conductor" is brought to the service equipment, Article 250.24(C)(1) still references back to table 250.66, which requires it as a % of the conductor size with a few exceptions that won't apply on a 100A service.
Now, I'd argue intent/letter of the law from an engineering perspective if the GEC was immediately sunk to ground 6' from the box and voltage drop not an issue, but the letter of the law appears very clear here.
