As I read yet another theologian attempt to defend the “non-factual” statement of Jesus that the mustard seed is not really the smallest seed, the answer became clear: It is a riddle.
Of all the seed of woman, Jesus was the least of them since he served them all.
The herbs were given to man to eat, while the grasses were given to animals. The greatest thing that man can eat is the body of Christ. So he is the greatest of the herbs.
The tree is, of course, the cross, and is the greatest tree.
Since the mountain is where you go to meet God, and the sea the masses of creation: If you have faith of Christ (the mustard seed) you can bring the place to meet God into the world.
To the Samaritan woman he said that though they worshiped God on the mountain, the day would come when they would worship him in Spirit. He moved the mountain into the hearts of men.
I think the parable has absolutely nothing to do with the literal mustard seed.