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