From: Rabbi Moshe Sofer
To: Rabbi Eliyahu Shra's
Rabbi Eliyahu asks whether eating sanctified offerings (kodshim) requires a kezayis or whether any amount suffices. The Chasam Sofer rules that both the prohibition against eating disqualified kodshim and the positive commandment of eating valid kodshim require a kezayis, resolving the difficulty of the showbread's small portions by explaining the Kohen Gadol fulfills the mitzvah with a full kezayis on behalf of all.
Source: Shu"t Chasam Sofer OC §49