In order to fulfill one’s obligation in the mitzvah of matzah, one must eat a k’zayis amount of matzah. This is because the Torah uses the wording of achilah (eating) when it commands the mitzvah of matzah. However, it is unclear how much marror must be eaten in order to fulfill one’s obligation in the mitzvah of marror. The Rush, in Arvei Pesachim, siman 25, says that the obligation is to eat a k’zayis of marror. The Rush says that the proof that we must eat this amount of marror is from the fact that the berachah that we make on the mitzvah of marror is “…al achilas marror,” and an achilah in halacha always constitutes eating a k’zayis amount.
The Shaagas Aryeh (siman 100) asks the following question on the Rush: There is a hekish that connects the mitzvah of matzah and marror. (It is this hekish that obligates women in the mitzvah of marror, for they would otherwise be exempt since it is a mitzvas assei she’hazman gramma). Based on that hekish we should learn that one must eat a k’zayis of marror, similar to the requirement of eating matzah. Why did the Rush prove that one must eat a k’zayis of marror only from the wording of the berachah? Why didn’t he prove it from this hekish?
Reb Chaim Soloveitchik (in his insights on Shas, known as his stencils) asks another question on the Rush. The Torah only considers something to have been eaten if it was a k’zayis amount. Even if the Torah does not use the word achilah, a k’zayis must still be eaten. We learn this from the prohibition to eat milk and meat together, whereby the Torah did not use the word achilah while writing the prohibition. Yet one will only transgress this prohibition when a k’zayis amount of the milk and meat mixture is eaten. So even if the Torah did not write the word achilah by the mitzvah of marror, we should still have to eat a k’zayis of marror since the mitzvah is to eat – and eating, by definition, is always at least a k’zayis amount. Why then did the Rush need to prove – from the wording of the berachah, and not from the fact that it is eaten – that one is required to eat a k’zayis of marror to fulfill his obligation?
Reb Chaim explains the source for the mitzvos of matzah and marror, and in doing so answers the abovementioned questions. He explains that there are two pesukim that discuss the mitzvah of matzah. One is “…al matzos u’merrorim yochluhu.” This pasuk refers to the mitzvah of eating the korban Pesach, and the Torah commands that the korban be eaten with matzah and marror. The second pasuk is “…ba’erev tochal matzos.” This pasuk is a separate commandment to eat matzah. The pasuk that refers to the mitzvah of eating the korban Pesach does not teach us that there is a separate mitzvah to eat matzah and marror; in fact the only obligation that is derived from that pasuk is to eat the korban Pesach. If not for the second pasuk (“ba’erev tochal matzos”) it would have sufficed to eat half a k’zayis of matzah, and half a k’zayis of marror together with the korban Pesach. The eating of the matzah and marror mentioned in this pasuk is only a precondition for fulfilling the mitzvah of korban Pesach. Since the eating of the matzah and marror are not separate mitzvos (as far as this pasuk is concerned) they do not require a k’zayis of each to be eaten.
Continuing his explanation, Reb Chaim says that we only find that there is a hekish between matzah and marror regarding the obligation to eat them with the korban Pesach. As mentioned above, that obligation is not a separate mitzvah and therefore does not require a k’zayis for each. The reason why the Rush could not have proven that one must eat a k’zayis of marror from the hekish between matzah and marror is because in the mitzvah of matzah to which marror is compared, there is no obligation to eat a k’zayis of matzah. The reason that one is required to eat a k’zayis of matzah is because of the other pasuk, “…ba’erev tochal matzos,” to which marror is not compared. Reb Chaim also explains that this too answers his question regarding basar v’chalav. For only when the actual mitzvah is to eat something or not to eat something does the Torah require a k’zayis. However, when the eating is not the actual mitzvah but rather a condition for another mitzvah (such as the obligation to eat marror together with the korban Pesach) we do not find that one must eat a k’zayis. Therefore the Rush was only able to bring a proof from the wording of the achilah berachah that we must eat a k’zayis of marror.