The kingdom of God encompasses the universe. It is unshakable and will last forever. It is the kingdom above all kingdoms. It is sovereign and supreme. Yet, Jesus surprisingly compares the biggest and greatest kingdom of all to the smallest seed of all — the mustard seed. Jesus' point is that the greatest things begin with the smallest things.
For instance, one of David's greatest sins was adultery with Bathsheba, which led to the subsequent murder of her husband Uriah. These sins eventually resulted in a civil war and the death of David's son, Absalom (see 2 Sm 12:11; 16:11, 22; 18:15). These "big sins" began with a little, impure glance (see 2 Sm 11:2).
Judas' great sin was betraying Jesus to the religious leaders, which led to Jesus' crucifixion. This worst sin may have begun with Judas stealing a little change from Jesus and the apostles (see Jn 12:6).
Elijah's greatest work was anointing Elisha. This anointing resulted in two other anointings which changed the whole country. This great work began when Elijah heard God in a tiny whispering sound (1 Kgs 19:12ff).
God's biggest and greatest work of all was Jesus' death, resurrection, and ascension followed by the coming of the Holy Spirit. This began when Mary conceived Jesus, Who became a human being smaller than my thumbnail. Don't scorn small beginnings (Zec 4:10).