Obviously, the parable does not mean the kingdom of God is literally "like a mustard seed" regardless of the relative size. Rather, it is meant to teach something by way of comparison. The main point of the teaching is found in comparing the mustard seed and what it grows into to the kingdom of God, not to other seeds.

Obviously, the parable does not mean the kingdom of God is literally "like a mustard seed" regardless of the relative size. Rather, it is meant to teach something by way of comparison. The main point of the teaching is found in comparing the mustard seed and the plant it grows to the kingdom of God, not to other seeds.

³⁰ And He was saying, “How should we liken the kingdom of God, or with what parable may we present it?— ³¹ as a seed of a mustard-plant, which when it is sown upon the soil is being smaller than all the seeds upon the soil. ³² And when it is sown, it goes-up and becomes larger than all the garden-plants. And it makes large branches so that the birds of the heaven are able to be nesting under the shade of it”. (Mark 4 DLNT)

First, small as mustard seed is a known rabbinic expression for the smallest amount possible (Tractate Berakhot 31a).¹ For the purpose of a parable, whose point is to use an example to teach a doctrine or principle, using expressions and illustrations which the audience accepts and understands is essential. In this case, the point is rabbinic Judaism teaches the mustard seed is the smallest amount of significance. (Droplets of blood smaller than a mustard do not make a woman unclean.)

The word translated "smaller" or "smallest" is μικρός which means small in size, space, age, quantity, rank or influence. Since the mustard seed is not the smallest in size, rank or influence is the meaning Jesus intends. The mustard seed is the least significant seed of all seeds sown. Despite its insignificance or humble rank it grew up, not to produce food, but to provide shelter for the birds of heaven. In other words, the kingdom of heaven is like the least significant seed on all the earth which when grown will provide a place to live.

_{1. Rabbi Barney Kasdan, Matthew Presents Yeshua, King Messiah, Lederer Books, 2011, p. 140}

Parable as Teaching

³⁰ And He was saying, “How should we liken the kingdom of God, or with what parable may we present it?— ³¹ as a seed of a mustard-plant, which when it is sown upon the soil is being smaller than all the seeds upon the soil. ³² And when it is sown, it goes-up and becomes larger than all the garden-plants. And it makes large branches so that the birds of the heaven are able to be nesting under the shade of it”. (Mark 4 DLNT)
³⁰ καὶ ἔλεγεν πῶς ὁμοιώσωμεν τὴν βασιλείαν τοῦ θεοῦ ἢ ἐν τίνι αὐτὴν παραβολῇ θῶμεν ³¹ὡς κόκκῳ σινάπεως ὃς ὅταν σπαρῇ ἐπὶ τῆς γῆς μικρότερον ὂν πάντων τῶν σπερμάτων τῶν ἐπὶ τῆς γῆς ³² καὶ ὅταν σπαρῇ ἀναβαίνει καὶ γίνεται μεῖζον πάντων τῶν λαχάνων καὶ ποιεῖ κλάδους μεγάλους ὥστε δύνασθαι ὑπὸ τὴν σκιὰν αὐτοῦ τὰ πετεινὰ τοῦ οὐρανοῦ κατασκηνοῦν

The mustard seed is used in a parable which is "the putting together of one thing along side of another by way of comparison or illustration." It can be an analogy:¹ In his commentary of Mark, D.E. Nineham explains the use and purpose of parables:

Parables were constantly used by the rabbis at and after the time of Our Lord, and the very numerous examples of their parables which have been preserved make it clear that they used them for the sole purpose of clarifying and driving home their teaching. When we observe the very close similarity of many of these rabbinic parables to Our Lord's - both in form and subject matter - it seems natural to suppose that he used parables in the same sort of way, and with the the same purpose, as the rabbis. That is to say, his general purpose in using parables was to make the truth as fully understood as possible; he may well have used parables, as the rabbis did, to provoke reflection and so bring his hears to a recognition of the truth.²

Obviously, the parable does not mean the kingdom of God is literally "like a mustard seed" regardless of the relative size. Rather, it is meant to teach something by way of comparison. The main point of the teaching is found in comparing the mustard seed and what it grows into to the kingdom of God, not to other seeds.

The Intended Meaning of μικρός
"Smaller than" in the parable is from μικρός which may convey small in size. It is also the word used to mean "small" in age, quantity, rank, or influence. It is used to describe Benjamin:

We are twelve brothers, sons of our father; the one is no more, and the smaller one is with our father today in the land of Canaan. (LXX-Genesis 42:32 NETS)
δώδεκα ἀδελφοί ἐσμεν υἱοὶ τοῦ πατρὸς ἡμῶν ὁ εἷς οὐχ ὑπάρχει ὁ δὲ μικρότερος μετὰ τοῦ πατρὸς ἡμῶν σήμερον ἐν γῇ Χανααν

Like the Greek μικρός, the Hebrew, קָטָן is used for small in size or age. It is possible Benjamin was the "smallest" as the NETS translated the passage, but that is the translator's speculation. Without question Benjamin was the youngest and the best reading of the passage understands age (i.e. youngest) is intended. A similar situation is found in the New Testament:

Whoever causes one of these little (μικρῶν) ones who believe in me to sin, it would be better for him if a great millstone were hung around his neck and he were thrown into the sea. (Mark 9:42 ESV)

As with Benjamin, it is possible to claim Jesus is using μικρός to describe size. Yet the passage is universally taken as reference to age or maturity. In Mark μικρός is also used to describe a distance (cf. Mark 14:35) and a period of time (cf. Mark 14:70). Therefore, if one wants to reject the possibility Jesus is using hyperbole or a rabbinic approach in order to make a comparison, there are other meanings of μικρός which should be considered before reaching a conclusion Jesus was wrong or there is some contradiction in His teaching.

Literal Truth in the Parable
"Small as mustard seed" is a rabbinic expression for the smallest amount possible (Tractate Berakhot 31a).³ The mustard seed was an accepted way to describe the smallest amount which is significant (i.e. droplets of blood smaller than a mustard do not make a woman unclean).⁴

Mark opens Chapter 4 with the parable of the sower (4:1-8). After explaining the purpose of parables (4:10-25), Jesus tells another parable of how a seed grows (4:26-29) before ending with the mustard seed. Here is a comparison of the seeds and soils in the three parables:

Parable         Soil             Seeds
Sower           Different soils  Same seed
Growth          Same soil        Same seed     
Mustard seed    Same soil        Different seeds

In the preceding parables, the focus is on a harvest from a single type of seed sown. In the last parable other seeds sown in the same soil are compared. The mustard seed does not result in a harvest; rather it produces shade which allow birds of heaven to nest.

The mustard seed can rightly be understood as the least significant seed to be sown in a garden. Not only is there no harvest, it grows up to shelter birds who, based on the other parables, will be a detriment to the garden. From the perspective of a planting a garden, the mustard seed ranks last among all seeds because there is no reason to want a mustard plant in the garden.⁵

<sub>1. D.E. Nineham, The Gospel of St. Mark, The Seabury Press, 1963, p. 126
2. Ibid., p. 128
3. Rabbi Barney Kasdan, Matthew Presents Yeshua, King Messiah, Lederer Books, 2011, p. 140
4. Smaller drops of blood are possible and do exist but they don't "matter" for this purpose.
5. The smallest known seed is of an orchid but even they would be more desirable than the mustard plant. They are attractive and do not provide a place for birds to nest.</sub>