Measure for Measure

Avakesh has an interesting post on shi'urim, the size requirements for mitzvos, i.e. how much matzah must one eat in order to fulfill the mtizvah (link)? In the late 18th century, R. Yechezkel Landau noticed that when comparing the Talmudic measurements based on the size of thumbs and those based on the size of eggs, there was a significant discrepancy. His conclusion was that the size of eggs had shrunk in half since the time of the Talmud (Tzelach, Pesachim 116b). Therefore, he concluded, we can calculate the approriate Torah measurements by using eggs and then doubling the size. Thus, the minimum amount of matzah is one olive's volume, which is half of a Talmudic egg. This would equal the volume of a full egg today. This was also accepted by his contemporary the Vilna Gaon (Ma'aseh Rav 74, 105) and has been accepted by many sages throughout the intervening years, most notably the Chazon Ish (Hilkhos Shabbos no. 39) and the Mishnah Berurah (271, Bi'ur Halakhah).

However, there have been those who disagree. Perhaps most prominent among them was R. Avraham Chaim Na'eh in his important work Shi'urei Torah, who defended what was at the time the custom in Jerusalem to use the smaller measure rather than the double measure. The Chasam Sofer (Orach Chaim 181) and the Aruch Ha-Shulkhan (Yoreh De'ah 324:10; Orach Chaim 168:13) also rule that we need not be strict on this.

Avakesh quotes a theory by Professor Avraham Yehudah Greenfield, published in the 1982 issue of Moriah (not Megadim), that if a thumb is measured slightly differently then the measurements are equivalent and there is no need to posit that eggs have shrunk and measurements must now be doubled:

Professor Greenberg responded with more and additional evidence and a reinterpretation of the Talmudic term on which everything was based. This term is rochav agodal.

This term is usually understood as thickness of the thumb at the knuckle; in other words, the distance across the width thumb. Professor Greenfeld argues that it should be taken as the thickness of the thumb from the nail to the opposite surface, which reduces rochav agodal by one half and removes the contradiction of Nodah B'Yehudah.

(See also here)

Interestingly, R. Eliezer Silver (d. 1968), in his book Tzemach Erez (Orach Chaim 486), posthumously published in 1975, offers the same theory. R. Shlomo Wahrman (Oros Ha-Pesach, no. 28), from whose essay on this subject most of the above sources were taken, quotes R. Silver as telling him that R. Meir Simcha of Dvinsk and the Rogatchover Gaon also agreed with this.