First, the kingdom of God begins as a small and insignificant movement. Second, the kingdom will grow rapidly and visibly. These two points are the foci of this parable and the ones that deserve attention exegetically. The observer has learned in the Markan account previous to this parable that the kingdom of God is very closely related to a seed and its growth. Seeds grow despite the sower.
Seeds grow wherever they are sown.
And seeds are meant for harvest. These are all important points to learn about seeds. And yet, the potential for growth is perhaps the most important, which may lead Mark to use this similitude as his climax of this discourse. As mentioned earlier there is no greater paradox of growth than the one present in the minuscule mustard seed and the enormous mustard bush. The acorn or walnut are seeds whose size bespeaks their potential.