NEC requires to size the EGC by using Table 250.122 as indicated in Section 250.122(A), and add requirements to comply with sections 250.4(A)(5) or (B)(4) for voltage drop consideration and to provide a return path for fault current respectively.
Section 250.122(B) added requirements when ungrounded conductors are increased in size, then the EGC requires to be increased in size in the same proportion according to the circular mil area of the ungrounded conductor.
Table 250.122 indicates the requirement for the minimum size of the EGC based on the rating or setting of the OCPD size, and provides OCPD ranges with corresponding EGC sizes for copper, and aluminum or copper-clad aluminum conductors.
It is very clear to understand the application of this requirements when the ungrounded conductor is oversized due to voltage drop, temperature correction, or by reducing the return path impedance to allow fault current to trip corresponding OCPD.
It is not to clear, the application when the ungrounded conductor is oversized due to energy conservation and/or purchasing convenience to get better price for the bucket.
NEC Table 250.122 shows OCPDs group sizes with a corresponding EGC. In other words, an specific EGC within a group of OCPD's (ie 101A to 200A) will require the same EGC size (ie #6 copper or #4 aluminum). It means that for cables which ampacities are between 101A to 200A or for ungrounded conductor sizes #2, #1, #1/0, #2/0, and #3/0, the same EGC #6 will comply.
For applications when the ungrounded conductors are oversized based on energy conservation or for purchasing convenience, meaning that there is not an issue with voltage drop, and or fault current return path to trip the OCPD. In this case, if I have to use a properly sized ungrounded conductor #1 AWG, but I will increase the size for convenience (call it energy conservation, etc), to go from #1 AWG to #2/0 AWG, then the same EGC #6 AWG will comply with NEC Section 250.122(A).