The mitzvah of Challah reminds us to recognize that everything we have in our lives comes from Hashem, and therefore, we give Him the first of our sustenance. This juxtaposes the idea of Avoda Zara, which represents someone who believes that there is another power in the world/nature that controls things. When one performs the mitzvah of Challah, that destroys the idea of Avodah Zara. That is why a professional baker who sees all the Hashgacha Pratis involved, in making his business successful and producing this piece of bread, needs a smaller amount of Challah/reminder. Whereas someone baking at home needs a larger piece/reminder.

לקו״ש י״ח -ה