Granted yeast puffs us out, doubling us in size, but in this particular parable we are warned of this, to guard against overinflated egos of the Pharisees, and the Political Herod. In this context Jesus is warning them of what will be coming, their dissent against God, as we continue to witness the growing agitation of those seeking the downfall of Jesus.
Yet the apostles did not understand this parable, thinking it was about them having only one loaf of bread. Jesus knew at that moment their hearts were hardened, after feeding a multitude of people with only a few loaves, the apostles still have yet to open their eyes and ears. Is it because it was right before their eyes and ears that made it difficult for them to understand. Just as when we are in a situation, when we walk away from it do we understand what it was about. Because of the Divine nature of Jesus, his parables to the apostles needed explaining, even though they witnessed what Jesus has done. Or was it that they were paying more attention to what was going on around them, than what Jesus was doing. Just as we do at times, we give more attention to what is going on to others when we are in God's house, than we do giving to God. Where even there we rise above ourselves as though we are leaven. God Bless you all with a smile in your heart.