All food Shi'urim are a k'Zayis except Tum'as Ochlin (which is a k'Beitzah).
This exception is owing to the change of Lashon applied by the Pasuk.
A confirmation of this may be drawn from Yom Kipur.
Question: What change of Lashon does the Pasuk use by Yom Kipur?
Answer: "Asher Lo Se'uneh" in place of "Asher Tochal".
Question: What differentiation do we make by Yom Kipur?
Answer: The Shi'ur is ke'Koseves.
Question: Why do we need confirmation from Yom Kipur?
Answer: We might otherwise attribute the change by Tum'as Ochlin to usage (syntax).
Question: How do we know that Tum'as Ochlin is a k'Beitzah?
Answer (R. Avahu citing R. Elazar): The Pasuk implies food which derives from food, i.e. a chicken-egg.
Question: Perhaps it means a kid-goat?
Answer: A kid-goat would not be called 'Ochel', since it still requires Shechitah.
Question: Perhaps the kid is a Ben Peku'ah?
Answer: It still requires Keri'ah.
Question: Perhaps the Pasuk refers to the egg of a large bird.
Answer: We apply the rule of minimum Chidush ('Tafasta ... ').
Question: Then we should use the egg of the smallest bird!?
Answer (R. Avahu himself): The Pasuk implies the most food which may be swallowed at one time, which Chazal assessed to be the volume of a chicken's egg.
CHANGES IN SHIURIM
(R. Elazar): One who eats Cheilev nowadays must record for himself the amount consumed, in case a future Beis Din will increase the Shiurim.
Question: What is meant by 'Yarbeh be'Shiurin'?
Answer: The Beis Din will obligate a Korban for a smaller k'Zayis.
Question: But then he will not have to bring a Korban, given the rule that a Chatas is only brought if one repents when he discovers that what he did not previously know!?
Answer: Rather, the Beis Din will obligate a Korban only for a larger k'Zayis.
Question: How could we have thought (in (c) above) that Yarbeh be'Shiurin might refer to their determining a smaller Shiur!?
Answer: The 'Yarbeh' refers to the increased number of Korbanos which he would thus be obligated to bring.
SHIURIM HALACHAH L'MOSHE MISINAI
(R. Yochanan): Shiurim and their Onshin are Halachah le'Moshe mi'Sinai.
Question: But the Onshin are written in the Torah!?
Answer: We mean the Shi'urim for incurring the Onshin (as supported by the Beraisa).
(Acheirim): The Shiurim for Onshin were established by the Beis Din of Osniel b. K'naz (Ya'avetz).
Question: But no one is permitted to introduce a Chidush!?
Answer: They had been forgotten and he restored them.
DRINKING A M'LO LUGMAV (MOUTHFUL)
(R. Yehudah citing Shmuel): We do not mean a true mouthful, but rather the amount which, if placed to one side of the mouth, will create the appearance of a mouth-full.
Question: But the Mishnah speaks of a mouthful ('M'lo Lugmav' [plural]which presumably is to be understood literally)!?
Answer: Understand it as ki'M'lo Lugmav.
Question: But we find the in the Beraisa the words 'M'lo Lugmav' to be literally a mouthful (as distinct from the opinion there 'ki'Melo Lugmav')?
Answer: The Beraisa is also to be understood as 'ki'Melo Lugmav'!
Question: Then how is Beis Hillel (M'lo Lugmav) distinct from R. Eliezer (ki'Melo Lugmav)?
Answer: Whether we mean the appearance of a cheekful without pressure (Beis Hillel) while R. Eliezer would not make the person Chayav until he drank a tight cheekful.
Question: If so, this should have been listed as one of the Kulos of Beis Shamai relative to Beis Hillel (since Beis Shamai obligates for a Revi'is and the relaxed cheekful of Beis Hillel is smaller) in Idayos!?
Answer: The question was asked regarding a giant person, such that Beis Shamai is Machmir there.
Question (the following three exchanges are between R. Zeira and Abaye): Why should we distinguish regarding liquids between individuals of different sizes, where we do not find such distinctions regarding food?
Answer: Chazal understood that a Koseves of food brings every person to a state of calm, whereas people vary regarding the amount of drink which brings calm.
Question: Could it be that every person is calmed with a ke'Koseves, even a giant?
Answer: Everybody is eased (from a moment of Inuy, and thus is Chayav) from a ke'Koseves, to a greater or lesser degree.
Question: How can we not differentiate between a Koseves of heavy, satisfying foods (meat) and foods with much less nutritional value (Lulvei Gefanim)?
Answer: Again, these will bring to differing levels of calm, but both will negate the Inuy of Yom Kipur.
Question (Rava): You have created a Kula in Yom Kipur by using the time frame for a k'Zayis (bi'Chedei Achilas P'ras) and applying it to a Koseves (such that a Koseves eaten in slightly more than bi'Chedei Achilas P'ras would be Patur)!?
Answer (Abaye): Chazal assessed that food eaten during this time frame calms, whereas more time does not bring calm.
Question (Rava): Chazal have created a Kula by Tum'as Ochlin in that a k'Zayis eaten bi'Chedei Achilas P'ras is considered Achilah, and yet to create Tum'ah in the body of the person (Tum'as Geviyah), it would take half a P'ras eaten in the same time frame (it is thus much harder to become Tamei)!
Answer (R. Papa): The Kula is because Tum'as Geviyah is de'Rabanan.
Question: But we find that R. Papa holds that Tum'as Geviyah is d'Oreisa!?
Answer: It is de'Rabanan, and the Pasuk is an Asmachta.
TZIRUF - FOODS COMBINING TO A SHIUR
(R. Papa): Umtza and its salt combine to make up a ke'Koseves.
This is true even though salt is not eaten, but since it is normally eaten on the raw meat, it is Mitztaref.
(Resh Lakish): Tzir combines with the vegetables on which it is found.
Question: Of course!?
Answer: We might have considered the Tzir as a liquid (which does not combine with food), and now we are taught that anything which renders a food more edible, combines with it.
ACHILAH GASAH ON YOM KIPUR
(Resh Lakish): One who eats Achilah Gasah is Patur.
Question: Why is this so?
Answer: Because one who is harming himself with his eating has not negated the dictate of "Lo Se'uneh".
Similarly, a non-Kohen who eats Terumah as Achilah Gasah pays the principle but not the Chomesh, since the Pasuk makes one liable for eating the Terumah, not for damaging oneself with it.