Any EGC is always based on Table 250.122.
If you look at Table 250.122 you will see the following wording.
Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)
There is no code section that allows reducing the EGC run with tap conductors other than 250.122(A) which tells us the EGC is not required to be larger than the ungrounded conductors.
