Gemara

(Mishnah - Chachamim): We never find that someone is liable for eating any amount!

Question: One is liable for eating an ant of any size!

Answer: That is because it is a full creation.

Makos 17a (Beraisa - R. Shimon): Also a wheat kernel is a Briyah!

R. Shimon addresses Chachamim according to their reasoning. I hold that even for flour, one is liable for any amount. You should admit to me about a wheat kernel, for it is a Briyah!

Chachamim say, an ant has a Neshamah, so it is a significant Briyah. A wheat kernel is not so important.

Berachos 38b (R. Chiya bar Aba): I saw R. Yochanan eat a salted (i.e. pickled) olive. He blessed before and after eating it.

Question (R. Yirmeyah): How could R. Yochanan bless after eating a salted olive? Since the pit was removed, it is less than a k'Zayis!

Answer (R. Zeira): One need not eat the volume of a big olive to make a Berachah Acharonah, it suffices to eat the volume of an average olive. R. Yochanan ate a big olive. Without the pit it was the volume of an average olive.

Chulin 119a: The law of Shomer (something attached to food that protects it, e.g. a lentil pod) applies to a Briyah of any size.

Rishonim

Rif and Rosh (Berachos 27a and 6:16): We learn from here that a Berachah Acharonah is needed only when there is a k'Zayis.

Rosh: The Yerushalmi brings another episode in which R. Chiya bar Aba was amazed that R. Yochanan blessed after eating an olive, for the pit was removed, and it was less than a k'Zayis. R. Yochanan blessed because it was a Briyah. If so, the same applies to a grape or a pomegranate pip. If this was the same episode that the Bavli brings, the Bavli argues and holds that even a Briyah must be a k'Zayis. The Bavli is primary. However, perhaps it was a different episode. In the case in the Yerushalmi, a full olive with the pit was brought in front of him. Even though he discarded the pit, since a full Briyah was in front of him and he benefitted from a full Briyah and ate all that is normally eaten from it, there is a Shi'ur. The Bavli could discuss when an olive without the pit was brought in front of him. One with Yir'as Shamayim will avoid the Safek and not eat (a Briyah) less than a k'Zayis.

R. Yonah (27b DH uvi'Gmara): It seems that the Bavli argues with the Yerushalmi. However, we can say that for something that is normally eaten without the pit, it must have a Shi'ur without the pit, for it is not considered a Briyah. If something is normally eaten with the pit, e.g. grape or a pomegranate pip or cherries, if he eats when it is considered a Briyah and he blesses afterwards even if in all there is less than a k'Zayis.

Pnei Yehoshua (Berachos 38b DH Iy): It seems that the Yerushalmi discusses grapes and pomegranate pips, and the episode in the Bavli was with an olive, because the law of Briyah applies only to the seven species. They are more important; a special Berachah was enacted for them. Some say that the Berachah me'Ein Shalosh is mid'Oraisa. Even according to those who say that it is mid'Rabanan, there is an Asmachta from a verse. Since the Torah wrote them in the singular ("Gefen u'Te'enah v'Rimon"), one is considered a Briyah. There is no source to say that a Briyah of other species is important. Normally, a Briyah is only a living animal with a Neshamah, e.g. for Bitul. A Neshamah is a Briyah, but a wheat kernel is not (Makos 17a). One opinion calls a wheat kernel a Briyah, but only for Tevel, for one wheat kernel exempts the entire pile. This explains our Gemara. If cooked foods keep their status, we understand why there is a Berachah Acharonah for a salted olive, even though it is less than a k'Zayis (for it is from the seven species). But if it loses its status, it is like another species. Why is there a Berachah Acharonah? The Gemara answered that one blesses afterwards Borei Nefashos, i.e. and this does not require a Shi'ur, like Tosfos said in the name of the Ri. This was R. Chiya bar Aba's Chidush! In the Yerushalmi, R. Yochanan asked 'don't you hold that the seven species require a Berachah Acharonah?!' R. Chiya was surprised because the pit was removed. R. Yochanan blessed because it was a Briyah, and the same applies to grapes and pomegranate pips. I.e. R. Yochanan initially meant that a Briyah of the seven species requires a Berachah Acharonah.

Rebuttal (Kaf ha'Chayim 210:17): Chulin 119a connotes that a lentil is a Briyah. The Acharonim say that Briyah applies to all species, including a small fish.

Tosfos (39a DH Batzar): The Bavli argues with the Yerushalmi. The Bavli is primary. R. Yosef says that they do not argue. The Bavli holds that it was a pitted olive. The Yerushalmi holds that it was complete with the pit, so one would bless even for a grape or a pomegranate pip.

Rambam (Hilchos Berachos 3:12): One who eats less than a k'Zayis does not say a Berachah Acharonah.

Tosfos (Sukah 26b DH v'Lo): We can say that in the case of the Yerushalmi, he ate a small olive with the pit. In all, it was less than a k'Zayis. The Bavli needed to say that it was a big olive, for Stam salted olives are pitted.

Poskim

Shulchan Aruch (OC 210:1): One who eats less than a k'Zayis of bread or other food does not bless afterwards at all. Some are unsure if we bless after something k'Briyaso, e.g. a grape or a pomegranate pip, even if it is less than a k'Zayis. Therefore, it is proper not to eat a Briyah less than a k'Zayis

Beis Yosef (DH veha'Rashba): The Rashba derives that the Bavli argues with the Yerushalmi, for it did not say that he blessed afterwards because it was a Briyah, rather, because it was a big olive. However, we can say that the Bavli holds that he did not eat the pit, and the Yerushalmi holds that he ate the pit. After it was cut or dissolved it is not considered a Briyah for any Isur in the Torah.

Beis Yosef (DH veha'Rif): The Rif and Rambam say that we bless afterwards for a k'Zayis. They do not mention a Briyah. They hold that the Bavli argues with the Yerushalmi. Even so, since so many Rabanan were unsure about this, one should not eat a Briyah less than a k'Zayis, like the Rosh, Tosfos, Hagahos Maimoniyos and Tur say.

Kaf ha'Chayim (20): If one wants to eat a Briyah less than a k'Zayis, he should cut off a small amount.

Rema: It is a Briyah only if he ate it like it is.

Magen Avraham (3): Chulin 119a connotes that one lentil is a Briyah, even though many grow in one pod.

Magen Avraham (4): If he ate what is inside the pit, this is called a Briyah, for he ate what is proper to eat. It is unreasonable to say that R. Yochanan ate the olive seed with its shell. Rather, he ate the inside and discarded the shell. Even if he ate the pit, we do not call a Briyah something that is not normally eaten, e.g. an olive seed, but it is normal to eat the seed of a grape or pomegranate. However, the Rashba holds that it is always a Briyah, and Tosfos says so even if he did not eat the seed. It seems that R. Yonah holds that R. Yochanan did not eat an olive, rather, another Briyah. The Shulchan Aruch discusses a grape or pomegranate pip, so the Rema's comment is according to all opinions. The Rosh agrees that if one did not eat a seed normally eaten, it is not considered that he ate a Briyah. The Beis Yosef (YD 101:6) bring from Hagahos Sha'arei Dura that if a little fell off in cooking, like normally happens, it is called a Briyah. This is like the Rosh.

Sha'ar ha'Tziyon (20): Also a small fish is a Briyah.

Mishnah Berurah (6): We bless after a Briyah because it is important. If it was diced, it is not a Briyah.

Mishnah Berurah (7): Even if the seed is not proper to eat, since he ate (all of) it as it is, he ate a Briyah.

Mishnah Berurah (8): All the more so, if part of the fruit was removed, it is not a Briyah. It seems that the same applies if a little fell off during cooking, like normally happens.

Rema (ibid): If he removed the seed, it is not called a Briyah.

Mishnah Berurah (9): Even if the seed is not proper to eat, since it was removed, the fruit is not called a Briyah. The Magen Avraham and Eliyahu Rabah hold that if he ate what is proper to eat inside the pit, this is called a Briyah.