The OP mentions a 25 amp circuit, so this is not a service, and we need not ask about GECs. I gather that there is a set of #12 ungrounded conductors with or without a neutral, and a #12 EGC, all run in EMT from the panel to a J-box. From that point, the wires run in open air, or in cable tray, or by some other "exposed" method, to the load. The OP wants to use a larger EGC in the exposed area. I think that is acceptable. The OP wants to not have to use that same larger EGC all the way back to the panel. I think that is acceptable as well.
What is not acceptable is the OP's statement that he plans to use a #12 EGC in a 25 amp circuit. You need a #10 minimum. Reference 250.122. What is also not acceptable, and this is an inference of mine and not a direct statement by the OP, is the use of #12 wires in a 25 amp circuit. I made that inference as a leap of logic from the OP's use of a #12 EGC. If that is indeed the OP's intent, then I would refer the OP to NEC article 240.4(D).