Peah has a minimum quantity (the Torah requires at least 1/60th) but no maximum amount (and all of the Peah that he separated would be exempt from separating Maaser).

Another Tanna taught that Peah, Bikurim and Re'ayon have no maximum or minimum.

Question: What is the practical difference between them, since this other Tanna agrees that Rabbinically, it does have a required minimum of 1/60th?

Answer: The opinion that said that Peah has a minimum but no maximum amount - if he gave less than the minimum, it takes effect (as the Torah allows even a tiny amount), but until he has added enough to it until it reaches the minimum, it is obligated in Maaser.

The opinion that said that Peah has no minimum and no maximum amount - whatever he already gave is exempt from Maaser (and with that he completes the Mitzvah) and whatever he adds later will be obligated in Maaser.

Question (R. Berachya): Why did the Mishnah not also list the following examples of things that do not have a minimum required quantity - the dirt added to the Sotah waters (when a woman might have been unfaithful); the ashes of the Parah Adumah (the red heifer's ashes that are added to spring water and then sprinkled on a person who is Tamei from a corpse); the spit of a Yevamah (as part of the Chalitzah ceremony, the widowed childless sister-in-law must spit on the ground); the blood of the bird offering of a Metzorah (that is put in the spring water)...?

Answer: The Mishnah only lists things that if he adds onto them, it is a Mitzvah.