I'm sorry I thought you were quoting Matthew.
Matthew 17:20 New International Version (NIV)
20 He replied, “Because you have so little faith. Truly I tell you, if you have faith as small as a mustard seed, you can say to this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you.”
It's not the size of the mustard see. They are tiny but the amount of faith required to let go of life as we know it and become born again completely helpless. That's the measure of faith required. We most surrender to Gods will and give up our own will in the process. We are the vessels we are not the potter. Have you ever seen a mustard seed? It's volume of faith is not the measure. The percentage of faith is.
I would add. Whose faith as the person thereof ? When the disciples were said to have little faith it was in respect to the faith of Christ that lived in them .Those who
know not the power of gospel are reckoned as having no faith .Not little none.
The faith that did work in them was not of their own selves. It will always be little compared to the source. He must increase as we decrease.
I think it is one of the reasons he spoke in parables to teach us of the unseen things of faith .Believing, is seeing the eternal.
There is a interesting discourse in Luke 9 . Where Jesus purposely hid the gospel understanding from the apostles. Again to teach them how to walk by faith as hearing God and not men. The Apostles who were following Jesus as the greatest in unbelief when not understanding. . . . immediately as if Jesus disappeared. They played who is the greatest alpha dog ,After the 3rd time he rebuked them and said; you know not the manner of spirit you are of. Which was the temporal corrupted spirit of natural unconverted man, no faith. Unlike the eternal Spirit of faith
By the faith of Christ he works to create mountains and moves men to believe.
But they understood not this saying, and it was hid from them, that they perceived it not: and they feared to ask him of that saying.
Then there arose a reasoning among them, which of them should be greatest.Luke9: 44-45