From The Gospel of Mark, by Mary Healy, commenting on Mark 4:26-34
It is as if Jesus is thinking aloud, searching for ways to help his listeners to grasp the mystery of the kingdom (see 4:11). Because the kingdom is a divine reality, it cannot be defined or contained in human categories. It can be understood only by using analogies, word pictures that force the listener to think and ponder at a deeper level.
Once again, the earthly reality most suitable as an analogy to the kingdom is, of all things, a tiny seed. In this third seed parable, the emphasis is on the seed’s smallness. For Jesus’ Jewish audience, the idea of the kingdom as a seed must have been quite a surprise. A more predictable comparison would be a mighty army (see Isa 13:4; Joel 2:11) or a cataclysmic earthquake (Isa 29:6). But no, the kingdom is like a mustard seed, which Jesus describes (using the device of hyperbole, or exaggeration, for effect) as the smallest of all the seeds on the earth. But once sown, it springs up and becomes the largest of plants (another hyperbole).
In mentioning large branches that shelter many birds, Jesus is evoking the Old Testament image of a lofty, shady tree, symbolizing an empire that grants protection to peoples of different races and tongues (Ezek 17:23; 31:6; Dan 4:9). The parable of the mustard seed thus points to the future worldwide reach of the kingdom of God. From its humble, inauspicious beginnings in Jesus’ itinerant preaching in Galilee with a small band of followers, the kingdom will mature to an immense tree in whom the Gentiles will find a home. This growth will not be due to human methods but to God’s hidden power. Jesus speaks with utter assurance of the future success of the kingdom, urging his disciples to persevere with hope and patience.