Sam, to reply to your second comment, I suggest that there is not necessarily a constant relationship between the size of the mothers and the size of the eggs. It could be that even though the hens were not so big, nevertheless the eggs were relatively bigger.

We find that Chazal say that Rivkah was three years old when she married Yitzchak. Children used to be mature at a much earlier age, and similarly it may be that eggs were bigger when they were laid even if the mothers were not, relatively speaking, so big.

Here are more sources from the Gemara about how people used to be bigger and stronger (and therefore possibly eggs as well):

1) The Gemara in Nidah (24b) tells us that Aba Shaul was the tallest person in his generation and that Rebbi Tarfon (of the next generation) only reached his shoulders. Rebbi Tarfon was the tallest person in his generation and Rebbi Meir reached his shoulders. This succesion continues until the second generation of the Amora'im and it seems that it was around that time that this phenomenon ceased.

2) The Gemara in Chulin (84a) tells us that Rebbi Eliezer ben Azaryah said that it is sufficient to eat meat only on Shabbos. Rav agreed with this, but Rebbi Yochanan (who was a Talmid of Rav) said that Rav was from a healthy family, while if we have a coin in our pocket we should buy meat in the stores whenever we can. (This Gemara is the source for the Rambam in Hilchos De'os 5:10, that it is enough for a healthy person to eat meat only on Shabbos.) Rav Nachman, who was a generation after Rebbi Yochanan, said that we should even borrow money in order to buy meat. Rashi (DH Amar) writes that the world was steadily becoming weaker, and they were not as healthy as in Rebbi Yochanan's times. We learn from this that people were not only becoming smaller, but also weaker.

3) Back to the question of the relationship between the size of the mother chickens and the size of the eggs: I have indeed seen reports that chickens now are four times bigger than they were sixty years ago. On the other hand, there are also reports that eggs are getting smaller, even the jumbo size. I do not claim to have any knowledge on the size of present-day chickens and eggs, but I suspect that if one looks into it one may find that the eggs are not getting bigger.

Kol Tuv,

Dovid Bloom

I am now going to attempt to show sources from Chazal for what I suggested in my first reply, that in the time of the Gemara children (and therefore possibly eggs also) were more highly developed at a young age than they are nowadays.

1) See Sanhedrin 69b, where the Gemara proves that in the early generations men used to father children at the age of 8. Tosfos there (69a, DH b'Yadu'a) writes that while in later generations Chazal detemined that pubic hair renders a girl as an adult at the age of 12 and a boy at the age of 13, in the earlier generations children would produce pubic hairs much earlier.

2) The Shitah Mekubetzes (Bava Kama 109b, DH Katan) in the name of the Rosh adds that not only in those days did they give birth at the age of 8, but they also had signs of adulthood ealier than this. He writes that it may be that they possessed large hairs and a beard before the age of 8 and, similarly, the women had breasts and much hair at such an age.

3) In fact, if we go back to the very beginning, we learn (Sanhedrin 38b) that on the eighth hour of the sixth day of Creation, after Chavah was created, it is said that "two went up to the bed, and four came down." Tosfos (DH v'Yardu) writes that Adam and Chavah went up to the bed and they came down with Kayin and his twin. Chavah gave birth instantly. At the very beginning of Creation, children were born without any lengthy gestation period at all.

Since we have seen how quickly children developed in the early times, this may make it easier for us to understand how eggs also used to be bigger.

Kol Tuv,

Dovid Bloom

1) Sam, I have found that the Teshuvot Chatam Sofer Orach Chaim #127 DH v'Nimtzah asks a similar question to yours. Chatam Sofer starts with what the Tzalach (by the Noda Biyehuda) writes in Pesachim end 116b DH Ho'eel that the eggs in our times are half the size that they were in the time of Chazal, but people are still the same size.

Chatam Sofer writes that this is a wondorous and amazing thing to say. However he then suggests that possibly it may be that in the places where Chazal lived, and possibly also where the Rif and the Rambam lived (North Africa and Spain) and maybe even in all the countries of the Sephardi sages, the chickens were different.

2)

Chatam Sofer bases his explanation on Chulin 62b where the Gemara relates that Ameimar taught that the "Tarnegolta d'Agma", the "chicken of the marsh", is forbidden to eat, because it had been observed that it stuck its fingernails into its prey ("Doreis"). Rashi DH Chazyuha writes that since we are not expert concerning different species of birds, if we see a new species, even though it posseses all the signs of a kosher bird, we still must be concerned that possibly it is "Doreis", since originally they thought the Tarnegolta d'Agma was kosher and only afterwards observed that it is Doreis. Rashi then states the important Halacha that one may only eat a bird if one possesses a Masoret for it. We may only eat it if our ancestors passed down to us that it is kosher.

3) CS writes that close to their times there was a bird called the "Pearl Hen" which possessed all the kosher signs, but they did not eat it because no Masoret was possessed for it. However there are countries where a Masoret is possessed on similar birds. The eggs of these birds were bigger than those of the kosher birds known where CS lived. Therefore CS suggests that the large eggs of the time of Chazal, and possibly those known in Sephardi countries, were laid by birds for which a Masoret was held in those countries that they are kosher, but this tradtion was lost in the Ashkenazi countries. This is why the eggs that the Tzalach saw were smaller.

4) However CS writes that this may answer why the Sephardi Gedolim do not mention any change in the size of eggs, but the problem is why did the Ashkenazi Gedolim of previous times, for instance the Rosh and afterwards the Tosfos Yomtov and many other Poskim, never make mention of the Halachic problems raised by the shrinking eggs? Why did nobody discuss the issue before the Tzalach?

5) Rav Chaim Naeh, who lived in Jerusalem around 70 years ago, suggests a different solution. In his sefer Shiurei Torah, vol. 1 Gate 3:5, he writes that eggs have not become smaller, but the point is that the size of eggs is very variable in different places in the world. For instance eggs in Egypt and Syria are very small. Where the Tzalach lived, in Prague, the eggs must also have been very small, but in other places in the world eggs are bigger and this can explain the Poskim who did not mention any change in size.

Dovid Bloom