Which indeed is the least of (is less than, Revised Version) all seeds; i.e. all those ordinarily sown in Palestine then. Instances of the proverbial use in the Talmuds of the size of a grain of mustard to express something very small, may be seen in Levy, s.
v. לדרח. But when it is grown, it is the greatest among herbs; it is greater than the herbs (Revised Version); i.e. than those which are usually called λάχανα. And becometh a tree, so that the birds of the air.
There is not necessarily any connotation of evil about these (cf. Matthew 13:4, Matthew 13:19); the thought is simply that those who are naturally outsiders are glad to come under cover of this tree.
Compare, for both thought and language, Daniel's description of the empire of Babylon (Daniel 4:12, Daniel 4:21), and Ezekiel's prophecy of the kingdom of Judah (Ezekiel 17:23). Come and lodge in the branches thereof.
Lodge ( κατασκηνοῖν); Matthew 8:20, note. In Palestine the goldfinches and linnets settle on the mustard in flocks.