ציטירנדיקער פּירוש אויף יונה 3:4

The Talmidim found this a compelling explanation considering that Yonah would likely have spoken to and interacted with Nineveh's inhabitants, more than the mere five words that are recorded in Yonah 3:4.

Aryeh Krischer suggests that perhaps the people of Nineveh were so receptive to Yonah’s message that it only took one day for the message to spread mouth-to-mouth across the whole city. Thus we are told “VaYaChel,” “And he began” (Yonah 3:4) – Yonah was in middle of his mission when it became a success. He did not even have the chance to finish it himself.

“In forty days Nineveh will be destroyed” proclaimed Yonah (3:4) throughout the streets of Nineveh. No mention, though, is made about the possibility of doing Teshuvah and avoiding the pending calamity. How could Yonah have omitted such a critical point?! While one could suggest that Sefer Yonah presents only a highlight of Yonah’s speech and that in actuality Yonah said much more, the classic commentaries do not adopt this approach.