From The Gospel of Matthew by Curtis Mitch and Edward Sri, commenting on Matthew 13:31-32:
The parable of the mustard seed builds on the previous one, showing that despite the enemy’s opposition to the kingdom, the harvest will yield tremendous results. . . . From this tiny seed, a great bush emerges. Jesus describes it as becoming so big that birds of the sky come and dwell in its branches—which recalls an Old Testament description of a great kingdom that gathers many nations as a large tree gathers birds who nest on its branches (Ezek 31:2–13; Dan 4:17–18). In particular, Ezekiel foretold that Israel would gather the nations like a mighty cedar that shelters the birds of the air (Ezek 17:22–24). Jesus uses this parable to show how his kingdom movement, despite its small beginnings, will become like the prophetic large tree gathering birds, fulfilling Israel’s mission to the nations as Ezek 17 foretold.