It seems, as always, there are differences of opinions concerning how much matzah to eat on Pesach. Apparently the dispute hinges on the difference between a ketzaya {Olive} and a Kebeitza {Egg}...
I found the following written on the topic of the size of kezayat...
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http://www.vbm-torah.org/archive/halak60/25matzah.docC. The Measurement of a Kezayit
There are several different aspects to this issue; we will deal with them one at a time:
The Relationship Between a "Kezayit" and a "Kebeitza"
An apparent contradiction within the Gemara seems to emerge regarding the size of a kezayit in proportion to a kebeitza. The Gemara in Yoma (80a) establishes that the human throat cannot swallow more than a single egg of a hen, and the Gemara in Keritut (14a) states that the human throat cannot contain more than two olives. The implication is, therefore, that a kebeitza - the size of an egg - is twice the size of a kezayit - the size of an olive. Other Gemarot, however, indicate otherwise. The Gemara in Eruvin (82b) concludes that the size of two average "meals" amounts to eighteen "gerogerot" - dates. Now from the mishna there in Eruvin it emerges that two average "meals" contain five and one-third kebeitzim. Thus, we must conclude that five and one-third kebeitzim contain eighteen dates, and therefore one date equals .296 of an egg, or a little less than a third.
Herein lies the problem. The Gemara in Shabbat (91a) records that Rava asked Rav Nachman, "What is the law if one threw an olive-sized piece of teruma into a house that was tamei?" [He replied: "The law in regards] to what? If in regards to Shabbat [a minimum of] the size of a date is required." The implication here is that a kezayit is smaller than a gerogerot Thus, if a date is around one-third of an egg, then an olive must be even smaller than that!
In other words, whereas this calculation renders a kezayit less than a third of a kebeitza, the inevitable conclusion we reached based on the Gemarot in Yoma and Keritut is that a kezayit equals one-half a kebeitza.
The Halakhic Ruling
One view in the Rishonim is that of Rabbenu Tam (Tosafot in Eruvin 80b, Yoma 80a, and Chulin 103b), the Raavya (Pesachim 525), Terumat Hadeshen (1:139), Maharil ("Seder Hahaggada") and other Rishonim. They maintain that a kezayit is half a kebeitza, and the aforementioned Gemara in Eruvin dealt with a different, smaller type of date (such as one without a pit).
The second opinion is that of the Rambam, who indicates that a kezayit equals around a third of a kebeitza (see Hilkhot Eruvin 1:9), as understood by several Acharonim (Magen Avraham 486:1, Peri Chadash). To resolve the aforementioned Gemara in Keritut according to this view, the Gera (to Shulchan Arukh O.C. 486) suggests that the Gemara there refers to eggs without the shells, while the Yeshuot Yaakov (O.C. 301) explains simply that the Gemara in Keritut argues on the Gemara in Eruvin. (Other answers have been offered, as well. A particularly intriguing view is that of the Rashba [Mishmeret Habayit 96a], who maintains that a kezayit is even smaller - less than a quarter of a kebeitza!)
The Shulchan Arukh (486) states, "The measurement of a 'kezayit' - some say that it equals a half a 'beitza.'" At first glance, it appears that the Shulchan Arukh rejects the view of the Rambam. It may be, however, that he actually viewed the Rambam's position - that a kezayit equals less than a third of kebeitza - as the most instinctively obvious opinion, whereas this amount corresponds with the size of olives in his day. He therefore felt the need to cite the dissenting view, that empirical evidence notwithstanding, a kezayit equals one-half a kebeitza. (See Rav Chayim Na'eh, "Shi'urin Shel Torah," p. 190, note 24.)
The Mishna Berura (486:1; 190:10; 456:2) rules that regarding Torah obligations one should follow the more stringent view and eat the size of half a kebeitza, while for issues involving rabbinically ordained requirements one may be lenient and use only a third of a beitza. When an issue of "berakha acharona" is at stake, one should recite a berakha only on the consumption of half a beitza, since we never recite a berakha when its obligation is in doubt. Likewise, whereas a berakha is recited on the mitzva of marror, one should follow the stringent view despite the fact that its obligation nowadays is rabbinic, unless one is ill and finds it difficult to eat a full half-beitza of marror.
All this regards the proportion of a kezayit to a kebeitza. We now turn our attention the size of a kebeitza itself.
The Measurement of a "Kebeitza"
Two basic questions surround the measurement of a "kebeitza" - whether or not to include the shell in the measurement, and the precise measurement of an average-sized egg with its shell.
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Topic: The "Evolution" of the Olive (Read 7323 times)