One explanation of this tradition is tied to the humility of Moses, who was “very humble (anav me’od), more than any person upon the face of the earth” (Num. 12:3). To record that Hashem called to him from the Tent of Meeting, Moses wanted to employ the same word used to describe Hashem’s summons of Balaam in Numbers 23:16: “Vayikar Hashem—the Lord met Balaam and put a word in his mouth. . .” The Lord, however, insisted that Moses use the word vayikra (and he called) rather than vayikar (and he met), which requires an aleph at the end. But Moses in his humility wrote the aleph extra small, not wishing to overstate the contrast with Balaam, or his own significance.
The Sefat Emet provides another interpretation. He writes that letters of reduced size in the Torah “seem connected to matters that depend upon action, the light of Torah flowing in a reduced and concentrated way into the actual doing of the mitsvah, as in ‘the commandment [mitsvah] is a candle’ (Prov. 6:23). This entire book [Leviticus] is about deeds.” (The Language of Truth: The Torah Commentary of the Sefat Emet, translated and interpreted by Arthur Green [Philadelphia, JPS, 1998].) In other words, the reduced aleph at the end of the word vayikra serves as a lens to focus the light of Torah on the deeds that are to be performed in the Mishkan. Exodus described the building of the Mishkan; Leviticus takes up the deeds performed within it.
At first these two interpretations of the small aleph might appear unrelated, but they are in fact deeply linked. Humility entails a focus on deeds. Instead of focusing on self, on getting ahead or getting even or getting fulfilled, the humble person focuses on the Master. Humility replaces all of the drama and complications of self-aggrandizement with the simple task of fulfilling the Master’s call. Moses left behind the little aleph 3500 years ago to remind us, in 21st century terms, that it’s not about us, but all about the One who calls.