Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field: Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof. Matthew 13:31-32

Jesus wasn’t claiming the mustard seed to be the smallest seed in the world, as it is obvious that he never added in the world to the statement.

The all seeds in the statement is limited to the seeds being commonly planted by the 1st century Jews in his days which his audience were aware of. This is made clear by the words in verse 31 …which a man took, and sowed in his field.