Jesus delivered 2 similar kingdom parables in Matt. 13 showing the expansive nature of the kingdom of God. The mustard seed starts out as small and not very impressive but will grow into the greatest of herbs. Then the nature of the leaven is to spread and grow and expand until it has consumed the whole loaf of bread. The nature of the kingdom of God should be to expand and grow, as we see in the book of Acts. If the kingdom is not expanding, we have to honestly examine ourselves and see the pattern of the early church where the kingdom was spreading like leaven to fill the whole earth.