At this point he is referring to the faith required for signs, and mentions a mustard seed is small in size, yet it is more potent than all others in its effects. So to bring out that the least degree of genuine faith is capable of great things, he mentioned the mustard seed.
-St. John Chrysostom, Homily 56 (PG 58, 549-50) Translated by Robert Charles Hill in "Spiritual Gems from the Gospel of Matthew", 4th Century


[The Lord] went on to mention mountains as well, and went even further: "Nothing will be impossible for you." Now, for your part, admire [the disciples'] sound values in not concealing their limitations, and the Spirit's power in gradually bringing on those without even a mustard seed in such a way that rivers and fountains of faith gush forth in them.
-St. John Chrysostom, Homily 57 (PG 58, 563) Translated by Robert Charles Hill found in "Spiritual Gems from the Gospel of Matthew", 4th Century