The assumption that big trees give big fruit, and small trees give small fruit is human logic. God, in His wisdom, designed the natural world according to God's plan. Something like, trees for timber are often huge, tall, but have small fruit. Small fruits that fall from trees would cause fewer fatalities for both humans and animals. Agricultural trees that give fruit for human consumption are dwarf, and are easy for fruit pickers to harvest. Flowers come from shrubs, and those who would like to enjoy their perfume must bend their own backs or go on their knees to enjoy the heavenly scent.
Jesus chose two agricultural images to talk about the mystery of God's kingdom, the image of a farmer who sowed seeds in the field, and the image of a mustard seed. Sowing was an annual activity of His society. These images are common and simple, and yet each has hidden meanings, something to do with their development. Farmers don't understand the process of the seeds' growing. They are amazed that a tiny mustard seed, small as it is, becomes a huge shrub. Both seeds are sowed in the ground by human hands. Both grow in a mysterious way. Both are useful for consumption. One is primarily for human consumption, and the other is good for animals.
Jesus employs the hidden meanings of a grain and a mustard seed to talk about God's Kingdom. The Kingdom of God in a person's heart begins with the seed, The Word of God, which is sown in the heart. The quality of a grain or mustard seed, light, weather, soil, and energy, all are helping a seed to grow. God's Word is perfect, and is above natural rules. A human's heart is conditioned by its environment, and that changes. Accordingly when a heart loves the values of his/her society, that heart is for that society. When a heart embraces God's love, that heart belongs to God.
The natural world is in a state of flux, and we are part of that natural world. We are unable to resist change, but must embrace change, because changing and growing go hand in hand. The heart of the matter is, there are different options for a person to choose from: God's way or the worldly way. A heart, that follows the way of the world, 'will follow' the way of the world. A heart that follows God's way belongs to God, and through that heart God will change the world. God's plant to change the world works through a human heart
It is not easy to understand a human heart, to saying nothing about the heart of the Divine! Life is a mystery. The mystery is a mixture of something learnable and something unknowable. Through God's love we slowly learn more about the mystery of life, and that takes a life time of learning, and growing. Christ's disciples are asked to be the signs of God's love for others. We are asked to do small, tiny tasks, something as small as a tiny mustard seed. We sow the seeds of justice, peace, and love. God, in His goodness, magnifies our tiny contribution to be enormous, useful for others.
Jesus says that God's kingdom on earth continues to grow in a mysterious way, invisible to our eyes, beyond our comprehension. Those who love God, humbly learn to believe in Jesus' teaching, that nothing on earth can stop the growing of God's kingdom, and that no images on earth are good enough to clearly explain God's kingdom. Our task is to learn continually, and be useful for others, and to be good stewards for our environment.