The Rambam writes (1:7) that even a "kol she'hu" of chameitz is prohibited because of the pasuk "lo ye'achel chamietz". The achronim all ask: achila in the Torah always means a k'zayis. There is a machlokes R' Yochanan and Reish Lakish (Yoma 85) whether less than the shiur of k'zayis (a "chatzi shiur") is assur min haTorah or only m'derabbanan, based either on a limud from a pasuk by cheilev or because of a sevara that each chatzi shiur is "chazi l'itztarufei", it can potentially combine with another chatzi to become a complete shiur. So here we have the Rambam (1) using the pasuk "lo ye'achel", with the term achila which means k'zayis, to prove the halacha of chatzi shiur; and (2) citing a makor for chatzi shiur that is unique to chameitz and distinct from the sugya in Yoma.
I unfortunately am pressed for time, so I will just be m'orer the difficulty for now. Maybe at some other point I can offer more.