The Twin Parables of the Mustard Seed and the Leaven (13:31-33).
These two parables have very different emphases. The emphasis in the first case is on the size to which it grew from small beginnings, from a tiny seed to a great tree with birds in its branches, from a tiny band of disciples to a world wide presence including both Jew and Gentile (Matthew 8:11; Matthew 10:18; Matthew 12:18; Matthew 12:21). The emphasis in the second case is on the leavening process whereby a little leaven permeates a whole batch of flour, indicating the invisible power that will be at work through the tiny band of disciples bringing about the final product in a larger ‘congregation of Israel', the new people of God. In this case the batch of flour indicates the potential Kingly Rule of Heaven.
Analysis.
a He set another parable before them, saying, “The kingly rule of heaven is like to a situation where a man took a grain of mustard seed, and sowed it in his field (Matthew 13:31).
b Which indeed is less than all seeds, but when it is grown, it is greater than the herbs, and becomes a tree, so that the birds of the heaven come and lodge in its branches (Matthew 13:32).
a He spoke another parable to them; “The kingly rule of heaven is like a situation where a woman took leaven, and hid it in three measures of meal, until it was all leavened” (Matthew 13:33).
Note that in ‘a' the grain of mustard seed is sown into the field, and in the parallel the leaven is hid in three measures of meal. Centrally in ‘b' the tiny mustard seed grows into a large ‘tree' in which the birds can come and lodge in its branches.