1) WHAT AMOUNT OF FOOD IS "MEKABEL TUM'AH"
OPINIONS: Rava asked Rav Nachman, "What is the Halachah in a case in which a person threw a k'Zayis of Terumah into a house which is Tamei?" The Gemara attempts to determine the law that Rava's question involved. His question could not have involved the laws of Shabbos, because the Shi'ur for Hotza'ah on Shabbos is a Grogeres (dried fig), and not a k'Zayis. Similarly, his question could not have involved the laws of Tum'ah, because the size of a k'Beitzah (egg) is necessary.
To what form of Tum'ah does the Gemara refer when it says that the Shi'ur of Tum'ah for food is a k'Beitzah?
(a) RASHI in Pesachim (33b, DH b'k'Beitzah) quotes the Toras Kohanim that implies that even the smallest piece of food is Mekabel Tum'ah mid'Oraisa. The Gemara here refers to the ability of one food to make other foods Tamei, as Rashi here explains (DH Iy l'Inyan Tum'ah).
(b) TOSFOS here (DH Iy l'Inyan Tum'ah) and in Pesachim (33b, DH l'Eimas) maintains that a piece of food that is less than a k'Beitzah is Mekabel Tum'ah mid'Rabanan, and the Toras Kohanim that Rashi cites is only an "Asmachta." Only food the size of a k'Beitzah is Mekabel Tum'ah mid'Oraisa, and this is how Tosfos explains the Gemara here. Tosfos points out that Rashi in Chulin (82a, DH v'Amar) retracted his opinion.
(c) The RASHBA maintains that food that is less than a k'Beitzah is not Mekabel Tum'ah at all, even mid'Rabanan.
2) ANALYZING RAVA'S QUESTION
QUESTIONS: The Gemara concludes that Rava's question to Rav Nachman involved a case in which a person throws a k'Zayis of Terumah into a house which is Tamei, and it falls onto another piece of food which is less than the size of a k'Beitzah. The question was whether the food, which becomes a Shi'ur with regard to Tum'ah, also becomes a Shi'ur with regard to Shabbos (so that the thrower is Chayav for transferring it from Reshus ha'Rabim to Reshus ha'Yachid).
There are a number of questions that we may ask on the words of the Gemara.
First, the Gemara earlier says that if a person carries out a piece of food that is less than the size of a Grogeres, and by the time he puts it down in another Reshus it has swelled to the size of a Grogeres, he is exempt from punishment. He is exempt because at the time that he carried it into Reshus ha'Rabim it was less than the minimum Shi'ur. Why, then, should a person be Chayav in the case of the Gemara here, if the object -- at the time of the Akirah (when he threw it) -- was less than the minimum Shi'ur (it was only a k'Zayis, and not a Grogeres)?
Second, why does the Gemara mention that a k'Zayis of Terumah was thrown into the Tamei house? Any Shi'ur of food that joins with the food in the house will be Tamei if the total size (after it lands) is a k'Beitzah!
Third, why does the food that was thrown have to be Terumah?
(a) The RASHBA and RITVA explain that even though the food was not actually a k'Beitzah at the time that he threw it, it is considered to have been a k'Beitzah at that time, because the act of throwing itself -- the Akirah -- caused the food to become bigger (that is, without the Akirah, it would not have become bigger). In the earlier case of the Gemara, when the food expanded to the size of a "Grogeres," its larger size was not at all related to the act of Akirah (it would have expanded no matter where it was). This answers the first question.
However, since the food that he threw was not in reality a k'Beitzah at the time of Akirah, it needs to have some additional significance at the time of the Akirah in order to be considered fully significant retroactively. For this reason it must be a k'Zayis of Terumah. By virtue of its size, and by virtue of being Terumah, it has significance (because a non-Kohen who eats it is Chayav Misah b'Yedei Shamayim). This factor, combined with the factor that it later becomes a k'Beitzah, makes it significant for Shabbos as well. This answers the second and third questions. (See also TOSFOS 91b, DH Pachos.)
(b) The Rishonim cite the RASHBAM who answers these three questions as follows. The reason the object was considered significant at the time it was thrown is not because at the end of its flight it landed on other food. Rather, it was considered significant at the time it was thrown for a different reason, independent of what happened later. (This answers the first question.) It was considered significant because it was a k'Zayis of Terumah Tehorah which is fit for a Kohen, and a non-Kohen who eats it is Chayav Misah b'Yedei Shamayim. This factor makes it significant enough to make one Chayav for Hotza'ah on Shabbos.
Why, then, does it need to join with other food to form a k'Beitzah? The Rashbam explains that the reason is because once it enters the house, it becomes Tamei and is no longer fit for a Kohen to eat, nor is a non-Kohen Chayav Misah for eating Terumah Teme'ah. Since it loses its importance as it enters the house, it needs another form of significance (i.e., it becomes a k'Beitzah in size) in order to be considered a significant Hanachah.