“the svarah is not a strong svarah”
There are two explanations for the svarah of chazi l’itztaryfai. According to the Tzlach ( and others) the Torah assered any amount since one might be oiver the issur kares by completing the shiur.
According to the Shagas Aryeh ( and others) the svarah is that if the torah gave kares for a full shiur then it is axiomatic that all of the components that got you to that shiur ( i.e. every little bite) must have been a cheftza shel issur.