Jerome

The kingdom of heaven is like a mustard seed, which a man took and sowed in his field. It is the smallest of all seeds, but when it has grown, it is greater than the herbs and becomes a tree, so that the birds of the air come and nest in its branches. It should not be a burden to the reader if we present all the parables. For those things that are obscure should be explained more fully, lest they be more entangled by excessive brevity than expounded. …

Commentary on Matthew ↗

Hilary of Poitiers

Now, in place of the herb, the preaching of the prophets was given to ailing Israel. But the birds of the air dwell in the branches of the tree that rises high above the ground. We see the apostles as branches stretched out by the power of Christ and overshadowing the world. The people of all nations fly there in the hope of life and, disquieted by the whirling winds—that is, by the breath and blowing force of the devil—they come to rest upon those tree branches.

Commentary on Matthew 13.4 ↗

Thomas Aquinas

Then follows its greatness. And first its greatness is set forth; secondly it is confirmed, at but when it is grown, i.e., when it has sprouted, it is greater than all herbs, because the evangelical teaching bore more fruit than the teaching of the law, since the teaching of the law bore fruit only among the Jews; hence it was said in Ps 147:20: he has not done in like manner to every nation, and his judgments he has not made manifest to them. …

Commentary on Matthew ↗