Sam2, kezayis is the bare minimum to be considered achila it is not the goal to strive for.
In other words there is a mitzva to eat matzah. period. We want to do ratzon Hashem so we eat matzah. How much is considered eating? So on that chazal tell us the bare minimum to be considered having eaten is a kezayis.
(I realize Imgetting repetetive thpugh its hard to convey in text)