Gd commands us to eat Matza on the first night of Pesach. May I eat just one crumb? No. How much must I eat? You must eat the volume of an olive. How much is that in grams or cubic millimetres? It is subject to a dispute between the rabbis.
See the link to Rabbi Slifkin's essay, on the left.
Why Only Eat a Single Kezayis? Rabbi Slifkin's blog HERE
The reaction of many people to my conclusions about the kezayis is one of shock, followed by the question: "So do you actually eat no more than such a tiny portion of matzah and maror?"
Why on earth would I, or anyone, eat no more than an olive-sized portion of matzah and maror? The mitzvah comes late at night, after a really long day, when I haven't eaten for hours. Any normal person will eat much more than an olive-sized portion!
The kezayis measurement is a minimum. Of course I may eat more if I want to, and I usually do. But those asking the question are thinking of the super-duper size kezayis that they eat after which they can hardly even look at more food. Halachah defines the minimum measure of eating in which anything less than that measurement is just not called eating. But normal eating obviously exceeds the bare minimum! Just as normal people do not use a sukkah of ten tefachim, the minimum height of a sukkah.
So people will be wondering if I eat no more than an olive-sized portion because they are thinking of an inflated kezayis and matzah and maror that are not really edible. Traditional matzah was soft and chewy, today's Ashkenazi matzah is more like concrete; traditional maror (wild lettuce, sowthistle, etc.) has been replaced by horseradish. This sort of eating is a challenge. People are so focused on eating the right quantity they no longer think about the mitzvah and the pleasure.
Imagine you are eating traditional matzah, and traditional maror (which was the normal hors d'oeuvre in antiquity), and a kezayisis a kezayis, why on earth would anyone eat no more than a kezayis?