Gemara

(Mishnah): The following are Metamei only if they are moist -- Zov, thick mucus or spit, a Sheretz, a Nevelah, and semen

If they are dry, but can become moist again if they are soaked, they are Metamei.

Menachos 54a (Mishnah): If calf meat swelled or if meat of an old animal shriveled, we judge them l'Kemos she'Hen.

(Rav): The text is Kemos she'Hen (according to their current volume).

(Shmuel): The text is l'Kemos she'Hen (based on the original volume).

Question (Beraisa): If calf meat was less than the Shi'ur for Tum'ah, and it swelled to the Shi'ur, it is retroactively Tahor (before it swelled) and Tamei for the future.

Answer: Mid'Rabanan it is Tamei for the future.

Question (against Shmuel - Beraisa): If a Shi'ur of meat shriveled to less than a Shi'ur, it is retroactively Tamei, and Tahor for the future.

Answer (Rabah): All agree that if a Shi'ur shriveled to less than a Shi'ur, we follow the present. If initially there was not a Shi'ur and it inflated to a Shi'ur, we follow the present mid'Rabanan. If a Shi'ur shriveled to less than a Shi'ur, and then returned to its original size, Shmuel holds that Dichuy applies to Isurim (and Tum'os. I.e. once it became less than a Shi'ur, it is never considered a Shi'ur again.) Rav disagrees.

Question (against Shmuel - Mishnah): If a Shi'ur of food or Neveilah was left in the sun and shriveled to less than a Shi'ur, it loses its Tum'ah. If it was left in the rain and returned to the Shi'ur, it is again Tamei (or one is liable for eating it).

Inference: Dichuy does not apply to Isurim!

Shabbos 91a - Question (Rava): If one was Motzi half a k'Grogeres to plant, and it inflated (to k'Grogeres), then he decided to eat it, what is the law?

Perhaps he is exempt, for he was not Motzi a Shi'ur for eating!

Or, perhaps since he would have been liable for his original intent (there was a Shi'ur for planting) had he not reconsidered, he is liable even if he reconsidered!

Question (Rava): If one was Motzi a Shi'ur, and it shriveled, then it inflated again to a Shi'ur, what is the law?

Regarding Shabbos, do we say that Yesh Dichuy?

These questions are not resolved.

Taharos 3:4 (Mishnah): If a Shi'ur of food or Tum'ah was left in the sun and shriveled, it is Tahor and one is exempt for it. If he then left in the rain and it inflated, it is Tamei and one is liable for it.

Rishonim

Rambam (Hilchos Ma'achalos Asuros 14:4): If a k'Zayis of Nevelah, Pigul, Nosar or similar Isurim shriveled in the sun, one who eats it is exempt. If he then left in the rain and it inflated, he is Chayav Kares or lashes. If it was initially less than a k'Zayis and it inflated to a k'Zayis, one may not eat it, but one is not lashed for it.

Ma'ase Roke'ach: The Rambam holds that the water can join to the Shi'ur, since initially it had a Shi'ur. He says similarly in Hilchos Avos ha'Tum'ah. I do not know why he did not say so in Hilchos Tum'as Ochlim.

Note: In Hilchos Tum'as Ochlim (4:6), the Rambam taught the first two laws, but not if it was initially less than a Shi'ur and inflated to a Shi'ur. The Kesef Mishneh says that he relied on what he wrote here.

Rambam (Hilchos Avos ha'Tum'ah 4:13): If less than a Shi'ur for Tum'ah of a Sheretz, Nevelah or Mes inflated to a Shi'ur, it is Tamei mid'Rabanan. If it was initially a Shi'ur, and shriveled, and inflated again to the Shi'ur, it is Tamei mid'Oraisa.

Perush ha'Mishnayos (Taharos 3:4): Our Mishnah teaches that we care only about the size. We are not concerned if it is porous or Mutzakim, not regarding Tum'ah, Isur or Heter.

Tosfos (Menachos 54a DH d'Me'ikara): Our Gemara concludes that (even) Rav follows the current volume only mid'Rabanan. In Shabbos, Rava asked about one who was Motzi half a Grogeres with intent to plant it, and it inflated, and he reconsidered to eat it. What was his question? Even if it swelled before he was Motzi, and he was Motzi from the beginning with intent to eat it, he is exempt, for mid'Oraisa it depends on the initial volume!

Poskim

Rema (OC 486:1): One must compress the pockets of air in the vegetables (Maror), and gauge a k'Zayis of the vegetables themselves, and not the air in between.

Gra (DH uv'Yerakos): Tosfos brings from the Yerushalmi that one must compress the gap, like it says in Uktzin. However, in Menachos we conclude that the stringency of meat that inflated is mid'Rabanan. The Rambam rules like this. Rabah tried to answer for Shmuel, but in any case Shmuel is refuted, so we need not say that it is only mid'Rabanan, and that Ein Dichuy in Isurim. The Gemara in Shabbos never settled whether Yesh Dichuy. This also answers the question of Tosfos in Menachos. In several places, it seems that the Halachah is Yesh Dichuy. However, R. Shimshon and the Bartenura (Taharos 3:4) say that Ein Dichuy. They explain our Sugya simply (that even in the conclusion, Ein Dichuy). Also the Ramban and Rashba in Shabbos toiled to answer Tosfos' question. They did not say that our Sugya is unlike the Halachah. In practice, I cannot be lenient against them.

Damesek Eliezer (3): The Gemara said that the stringency of meat that inflated is mid'Rabanan. I.e. mid'Oraisa it depends on the initial size. If so, one is not Yotzei with Matzah that was less than a k'Zayis and inflated. However, since in any case Shmuel is refuted, we can explain simply that Rav and Shmuel argue about whether we follow the initial or current size, unlike Rabah. Rava's question in Shabbos assumes that we follow the current size. However, Tosfos, the Ramban, and Rashba toiled to resolve Rava with Rabah. This shows that they hold like Rabah. The Gra did not want to be lenient against them.

Aruch ha'Shulchan (YD 85:15): Regarding Korbanos, Yesh Dichuy, but regarding Isurim, it is clear to the Gemara that we are stringent, and Ein Dichuy. The Gemara was unsure whether Dichuy applies to a Mitzvas Aseh to be lenient (to exempt from the Mitzvah).

Keren Orah (Menachos 54a DH u'Masik): The Gemara said that it depends on Dichuy. This is not the Dichuy we discuss regarding Kodshim and Mitzvos. Why should Dichuy apply to Isurim, since now there is a Shi'ur?! Rather, they argue about reasoning. Shmuel holds that once it lost its Shi'ur, it is as if it never had a Shi'ur. Completion through inflation is not completion. Rav holds that since initially it had a Shi'ur, it can return to the Shi'ur. The Ri Korkus (Avos ha'Tum'ah 4:13) asked that in Shabbos, we never resolved about a Shi'ur that shriveled between Akirah and Hanachah. Here we hold that Ein Dichuy! I say that Dichuy applies there, for there was a time when one would be exempt for the Hotza'ah, but here Dichuy does not apply.

Sefas Emes (Menachos 54b DH Tiyuvta): Since Shmuel is refuted, why didn't Rabah explain simply, that they argue about whether we follow the initial or current state? We can say that the Mishnah discusses anything that inflated in the rain, not only if initially it had a Shi'ur and shriveled! However, the Rambam rules like Rabah. It seems that based on his own reasoning, Rabah said that Rav and Shmuel agree in the other two cases. However, why did the Rambam bring the Mishnah simply, without explaining that it is mid'Rabanan to follow the volume after it inflated?!

Chasam Sofer: The Rambam holds that something that inflated due to rain is considered a Shi'ur only mid'Rabanan regarding Tum'ah and Isur. All the more so one is not Yotzei a Mitzvah (e.g. eating Matzah) in such a case!

Shulchan Aruch (210:1): One who eats less than a k'Zayis does not bless afterwards.

Mishnah Berurah (1): If a k'Zayis shriveled, one does not bless after it unless it inflated again.

Shulchan Aruch (645:5): If most of the leaves of a Lulav (Rema - or its spine) dried, it is Pasul. Dry is when it ceases to be green, and whitens.

Aruch ha'Shulchan (15): If some of the greenness returns after it is soaked in water, this is not called dry, and it is Kosher. We see that this happens. This applies to everything. If Dam Nidah dried, and it returns to be moist when it is soaked, it is considered moist (YD 188:4). Regarding anything that changed, we follow how it is now, like it says in Uktzin and Menachos. If it returned to be green before Yom Tov, Dichuy does not apply at all. Even if it returned to be green on Yom Tov, we hold that Dichuy does not apply to Mitzvos.

Gan ha'Melech (122): The Rambam brings the Mishnah in Taharos in Hilchos Ma'achalos Asuros, Hilchos Avos ha'Tum'ah and Hilchos Tum'as Ochlim 4:6. We learn from the Mishnah to a Berachah Acharonah. If something was a k'Zayis and shriveled to less than k'Zayis, we do not bless afterwards Borei Nefashos. If later it inflated again to a Shi'ur, one blesses afterwards. If regarding Tum'ah and Isur we judge it based on how it is in front of us, all the more so regarding Berachah Acharonah, which is only mid'Rabanan! However, the Rambam says that if less than k'Zayis inflated to a k'Zayis, one is not lashed for it, and it is not Tamei. Menachos 54a connotes like this. If initially it did not have a Shi'ur, and it inflated to a Shi'ur, mid'Rabanan (it is as if it has a Shi'ur). Since this is only a stringency, for a Berachah Acharonah we should not be stringent about a matter whose source is mid'Rabanan. This is like contradictory stringencies, for the Berachah itself is an enactment mid'Rabanan. Also, there is concern for a Berachah l'Vatalah, which transgresses "Lo Sisa (Es Shem Hash-m Elokecha la'Shav)." However, it seems that one blesses. We should not add to the argument. The Rosh (Berachos 6:16) is unsure whether there is a Shi'ur for Borei Nefashos. Perhaps one blesses on any amount. Tosfos (Berachos 39a DH Batzar) brings so in the name of the Ri. See the Beis Yosef (210 DH v'Chen, who says that the Rambam and Rif disagree.) We can say that they argue only about less than a Shi'ur, but if it inflated and now it has a Shi'ur, all agree that we bless after it. Also, perhaps something that inflated is Asur only mid'Rabanan when it inflated by itself, e.g. he left it in the rain. Something that it is normal to soak it and it inflates and this is how we benefit from it, it is considered to have a Shi'ur in every respect, both for Isur and Tum'ah. This is mid'Oraisa. It is not a mere stringency mid'Rabanan. I have proofs for this, I do not need them for a Berachah Acharonah. For this, even if he left it in the rain, he blesses afterwards.

Aruch ha'Shulchan (YD 85:17): Even though if something shriveled and inflated to a Shi'ur, he is liable, this is only if there is no hollow at all. This is even for something that normally has hollows, e.g. wheat bread. Inflation helps only to spread out what shriveled, but not if it makes hollows. Uktzin 2:8 proves this. It says we follow the current volume, but if there is a hollow, we must compress it. Also the Yerushalmi says so.