R. Yochanan (Berakhot 52b) states that it is permissible to destroy bread crumbs that are smaller than a k’zayit (although Tosafot notes such behavior is spiritually risky). The Magen Avraham (O.C. 180:3) implies that when less than a k’zayit is involved, there is no prohibition of “bal tashchit” (See Resp. Birkat Ephraim, 8). The Levush (O.C. 180:3) states that these crumbs, because of their size, are not considered food (apparently assuming that is the significance of the measurement of k’zayit) and thus not subject to the prohibition of bizayon okhlim (disrespecting food). The Orach Meisharim (Shulchan Arukh L’Midot) explains that the practical cost of protecting this type of food is more than the food is worth. The Birkei Yosef (#3) rules that the permissibility to destroy them only applies if the total sum is less than a k’zayit; if they add up together to a k’zayit, it is forbidden.

The Sefer HaChinukh, however, applies bal tashchit even to a mustard seed (see Resp. Minchat Elazar, IV, 61:2).Their dispute may hinge on the question of the nature of the prohibition of bal tashchit: is it a function of negating benefit from the world, or of the deleterious quality of destructive behavior. If the latter is the dominant element, it may not matter that the destructiveness is applied to an insignificant amount. If, however, it is the former, it may be that if the amount is less than can be noticed as a significant benefit, such as less than a k’zayit, there may be no prohibition. Similarly, the Resp. Chavalim B’N’imim considers whether there is relevance to the standard of “shaveh prutah” which is the measurement of monetary benefit (see Resp. Hit’orr’rut Teshuvah (IV, 35, and Ikvei Sofer).

R. Yosef Engel (Resp. Ben Porat, I, 6:6) notes that it is interesting that R. Yochanan permits bal tashchit less than a k’zayit, when he is the opinion in the Talmud (Yoma 73b) that considers eating less than a k’zayit of forbidden food (chatzi shiur) to be biblically prohibited. He suggests that the essence of the prohibition of bal tashchit is destroying an item of significance; this is indicated by the Torah singling out “a tree of edible [fruit]” in the formulation. Thus, when the item is insignificant, there is simply no prohibition at all.