Parshat Vayikra starts with the word ויקרא written with a small א. The word ויקר means it happened by chance. The Baal Haturim writes that when Hashem called out to Bilam, the Pasuk says ויקר, since he only deserved to be called by Hashem by chance. However, the voice that called out to Moshe was not by chance; it was the same voice that Bnai Yisrael could not tolerate listening to at Har Sinai. Therefore, Hashem allowed Moshe to write ויקרא, but Moshe was too modest to write it, so he wrote it with a small א.
The קרל"ה says that the extra ink that Moshe saved by writing a small א was divine radiance flowing from Moshe’s forehead as a reward for his modesty.
Rabbi Yaakov Menken says that Moshe’s action serves as a lesson for us. Just as Moshe tried to limit his honor, we should limit our honor. But the same way Moshe’s humility honored him, our humility should honor us. We believe that “He who runs away from honor, honor will chase him.”