Isaac inherited God’s promises to Abraham. In Genesis 26:12 we’re told that he planted crops and in that same year "reaped a hundredfold because Yahweh blessed him”.
When Jesus spoke of the fruitfulness of his word in people’s lives, he spoke of it multiplying thirty, sixty or even a hundred times.
Maybe he had in mind God’s blessing of the patriarchs and his promises to them, which were now coming true through his transforming word?