For most mitsvos it is easy to exceed a shiur, which makes the shiur largely irrelevant. For example, one is required to eat a kazais of hallah in the sukkah but unless you have some serious medical problem, that’s no big deal. Eating a kazais of matsa and marror can be a problem except for those of us who love to eat dry crackers and hot horse raddish, in which case the seder cuisine is quite unchallenging.
