The Mustard Seed and the Church
Our Lord compared himself to a mustard seed, for although he was the God of glory and eternal majesty he became completely small in that he willed to be born of a virgin in the body of a child. Thus he was placed in the earth when his body was laid in the tomb but, when he had risen from the dead through his glorious resurrection, he grew great on earth until he became a tree in whose branches nest the birds of the air.
This tree stands for the Church, which Christ’s death has raised up in glory. As for its branches, these must be understood as the apostles since, just as branches are the tree’s natural adornment, so the apostles are the adornment of Christ’s Church through the beauty of grace they have received. And on these branches, as we know, dwell the birds of the air. Allegorically speaking, the birds of the air designate ourselves who, coming to Christ’s Church, perch on the teaching of the apostles like birds on the branch.
— Saint Chromatius of Aquilaea (+407), Sermon 30, 2 (trans. SC 164, p. 137)
The Wikipedia article on Christ’s parable notes that the black mustard plant (thought to be the one used in the parable) grows to be a small woody shrub, not a mighty tree. It was known in ancient times for its invasive properties, “spreading like a weed” wherever it is planted. Pliny the Elder wrote:
Mustard … is extremely beneficial for the health. It grows entirely wild, though it is improved by being transplanted: but on the other hand when it has once been sown it is scarcely possible to get the place free of it, as the seed when it falls germinates at once. (quoted in the Wikipedia article)
Since Christ might easily have used an oak or a cedar to make his point, perhaps he chose the mustard plant partly for its ability to spread — like His Kingdom — despite man‘s efforts to stop or control it.