[Chalah] for fire has a Shi'ur, for it is mid'Oraisa. The one for a Kohen has no Shi'ur, for it is mid'Rabanan.

Question: He should separate for fire (mid'Oraisa), and not for a Kohen!

Answer: This is lest people say 'we saw Tahor [Chalah, which is like] Terumah (of Eretz Yisrael) burned. (Now that they must separate two Chalos, it is known that it is of Chutz la'Aretz - GRA.)

Question: (In the Seifa, from the river and Amnom and outside, Chalah for fire has no Shi'ur, and for a Kohen has a Shi'ur.) He should separate for a Kohen, and not for fire!

Answer: This is lest people say 'we saw Tamei Terumah eaten.' Since he separates two Chalos, it is known that it is of Chutz la'Aretz - GRA.)

Both of these are mid'Rabanan. It is better to increase the one that is eaten (it has a Shi'ur), and not to increase the one that is burned.