Jesus compares the Kingdom of Heaven to a mustard seed, which to be strictly accurate, is not the smallest of seeds, but in the east, the mustard seed was proverbial for its smallness. For example, the Jews talked of a drop of blood as small as a mustard seed; or, if they were talking of some tiny breach of the ceremonial law, they would speak of a defilement as small as a mustard seed and Jesus himself used the phrase in this way when he spoke of faith as a grain of mustard seed. So it was an expression or a figure of speech.

Jesus uses this and the parable of leaven to make a few points. The first is that The Kingdom of God begins very small...