This year at the community seder I was handed a packaged matza with the instruction to eat it. We later discovered on the package the text "this matza is equivalent to a kezayit and must be eaten whole". We all agreed that kezayit should mean "like an olive", made some jokes about monster olives and I forgot about it until I came across a fascinating article by Natan Slifkin about The Evolution of the Olive. I recommend reading the whole article, but here are some interesting points.
How do we define a kezayit? Early ashkenazi rabbis arrived at measurements that revolve around 1/3 of an egg. Why did the rabbis even have to discuss the measurements of a kezayit? Why didn’t they simply assume it is just what it says, the size of an olive? The answer is really simple, the rabbis had never seen an olive, as olives do not grow in northern Europe.
R. Karo in the Shulchan Aruch (Orach Chaim 486:1) brings an opinion that a kezayit is half an egg. This is not a ruling, just a vague "some say", but it still is treated as a ruling by many later authorities. R. Karo and the later authorities knew olives. Faced with the evidence of olives smaller than half an egg, someone came up with the idea that olives were bigger in old times (see "decline of generations"). Along the same lines goes another reasoning about eggs based on some egg-thumb comparisons, which postulates that talmudic eggs were double the size of our eggs.
So where does that leave us, how much is a kezayit in cc? An egg today measures about 50-60cc. A popular opinion for the size of kezayit is 28.8cc (based on half an egg). Another proposed measurement is 17cc (a third of a smaller egg). Yet another proposed measure is 50cc (remember the double-sized talmudic eggs). And there are still people around who maintain a kezayit is just that – the volume of one of today’s olives (5-6cc).
Still the question remains, a kezayit is a three-dimensional measure for volume. Matzot are two-dimensional. So how many pieces of olive-sized matza parts do I have to stack in order to arrive at the volumen of an olive? To avoid such difficulties, a kezayit can be given in terms of weight. I’m not sure where the specific number comes from, but it is given as 30g in the article. That is definitely more than an olive (the ones I weighed had 5-6g). I tried to weigh some eggs as well, I had small eggs, they were between 47g and 51g. So 30g could be half of a bigger egg.
One slice of my machine-made matza weighs about 35g. So you would need to eat nearly all of it for a kezayit of 30g. In other places, I found 24g or two-thirds of a machine-made matza as a specification of kezayit for the seder (see e.g., Selected Halachos Related to Parshas Vayikra at Torah.org or Laws of the Seder at Aish haTorah). Actually, I think you also need to eat a kezayit of bitter herbs for maror – why does nobody insist on that (I had one leaf of Belgian endive as had everybody else, definitely not 30g)?
I think this is a fascinating example of how some small detail gets more and more complicated over the centuries. If the early ashkenazi rabbis had known olives, it would probably be very simple to determine kezayit: Just take an olive. There is no basis in history, archeology or botany that olives had a different size in old days and if anything eggs were smaller 2000 years ago. Plus, even if olives or eggs were different, the idea that we are supposed to measure what we eat based on the olives of the old days and not our own olives is still questionable. But somehow we insist on eating so much matza at the seder that we are fed up with it for the rest of the week. Now go and explain to my mother why.