Ok. So you have a load, say it is 50 amps. We use THHN. We are limited to the 75 degree rating of the termination provisions for the devices to be used even though THHN is rated as 90 degree C. We find that we can use #8.
Conductor has been selected, #8 THHN.
For some reason, any reason, or no good reason at all, we install a #6 for one of the ungrounded conductors. We have increased the size. We have provided the capacity for more fault current to flow than the original conductor would have allowed. Per 250.122(B), we need to proportionately increase the size of the EGC.
For ungrounded: could've use 16510 cmills, did use 26240. Increased by 1.5893%.
For EGC: per 250.122(A) for 50 amps, could've been #10 (10380 cmills), but per 250.122 increase by 1.5893% to 16497 cmills, so must be larger than #10. Use #8 EGC.
Certainly not perfect, if the load is allowed to be protected at 60 amps, we replace the OCP and presto! We've not increased the size all of a sudden. The section needs work, which is why they should throw T250.122 out and fashion a new one after the logic of 250.66.