Today’s daf, Menachot 26b, records a debate between R. Yehoshua ben Levi and R. Yochanan whether a kometz, we can assume that even the smallest handbreadth (of the Kohen with the smallest hands), holds at least the volume of two-olives. If they are debating this point, it is fair to assume that the size of a small handbreadth is right on the margins of the the volume of two olives – hence the debate. Thus, it can serve as a good estimate for the size of two olives and, by simple division, the size of one olive.
So, it is only reasonable to ask – if the volume of a kometz, the amount contained under the middle three fingers when folded down unto the palm of the hand, is the size of 2 kezeitim, and twice the size of a kezayit, wouldn’t that make a kezayit pretty small? I mean, wouldn’t that make a kezayit about the size of an olive?! And then wouldn’t that mean that when it comes to mitzvot of eating – particularly the upcoming mitzvah of eating matzah – that even a pretty moderate amount would count as a kezayit?
The obvious answer to these questions is yes – a kezayit is only the size of an actual olive. [It is actually quite amusing to look at some of the handbooks that give measurements for how much of a given food constitutes a kezyait, and to see how many olives it requires to make a kezayit!] It is true there is some debate by bread and the like, whether we include the air pockets in the measurement, but in the case of matzah the measurement should be pretty straightforward, and it should be considerably less than many people assume.
How then did the size of a kizayit grow? This topic is explored at length, and in a clear and lucidly presented manner, by R. Natan Slifkin, in his article “The Evolution of the Olive.” One fascinating point that R. Slifkin makes is that while Sefardi Rishonim ate olives, and spoke only about using the measure of an actual olive, the olive was unknown to Ashkenazi Rishonim, as they attest to directly. Thus, it is in Ashkenazi Rishonim that we begin to find the discussion of estimating the size of an olive, and adopting large estimates because it was not possible to directly determine the size. You can find this article on his website, and also under Resources on this blog.