Yeast is often used in the Bible as a picture of sin or false doctrine because it so easily contaminates everything (1 Corinthians 5:6-7).  But here, Jesus uses the little mould to represent the gospel.  In the same way that yeast is ineffective if it stays away from the dough, so the gospel has no power unless it is mixed into the world.  30kg of flour is a large batch, but only 1% of yeast is required to make the bread rise.  Jesus wanted His hearers to imagine the energy expended in kneading the yeast into it.  

The woman would have worked very hard as she stretched and pounded the moist dough.  Inadequate effort at that stage would result in an inconsistent texture to the bread.  However, the rising of the dough was not due to the strength or skill of the woman, but the inherent power of the yeast to expand the whole batch of dough.  The biochemistry of fermentation is not under the control of the baker; it comes from the way God has designed yeast to function as a living organism.
 
In the same way, the gospel's power is in the Word of God which has intrinsic power to change people's lives by its self-replicating truth.  Yet the baker (first the apostles and now us in the 21st Century church) must work very hard to get the yeast to the right place.  When it is there, the yeast takes over: it multiplies and ferments until the loaf is well risen and ready for the oven.  The gospel is like that.  It is our job to get it into our community (1 Corinthians 15:10), or at least the little corner which is the world where we are.  God's job is to awaken their hearts to its truth.
 
But it is not easy.  Kneading the seeds of gospel truth into a society is hard work.  But do not abandon the task because of that. Once the gospel is placed into people, it has the power to work its truth into their minds and hearts and start to change them from the inside.  We cannot see that, any more than we can see with the naked eye how the bread rises; but we see its effects in changed lives and families and communities.  So, do not be afraid or ashamed to work hard in gospel ministry because the power that God releases is far greater than anything we can put in.