What the parable illustrates is that the kingdom of God, which will be established on the whole earth, had a small beginning but will have a very great end.
A contrast as wide as the very small beginning of a mustard seed, which when planted, that is planted with a purpose, grows and in its final state, becomes many times larger than the size of the mustard granite, to become a vegetable that gives shade and coverage to the birds.
That is the essence of the illustration, a small beginning and a big ending. This same contrast and concept of the parable can be seen in the dream of King Nebuchadnezzar, in which the stone was made a great mountain that filled the whole earth, illustrating that God will raise on the earth a kingdom that will never be destroyed (see King Nebuchadnezzar's dream and the interpretation of the prophet Daniel in Daniel 2: 35 and 44).
The establishment of the kingdom of God on earth, has already had a beginning, a beginning that can be perceived as small, when God Himself made flesh and bones in Jesus the Son of God (God the Son), came into the world, being born in a humble manger, very far from the honors that will correspond when he returns to earth to establish his kingdom as Lord of Lords and King of Kings (as King over all the kings of the nations of the world).