"'Consider carefully what you hear,' he continued. 'With the measure you use, it will be measured to you - and even more. Whoever has will be given more; whoever does not have, even what he has will be taken from him.'" NIV translation
Jesus was speaking here about faith and the mystery of how we grow to mature in Christ. If you want to grow to be a great man/woman of faith, then you must exercise what you have. As you act more in faith, you will find that you understand more about God and His provision for you. You start where you are at and do what he asks you to do knowing that He will not ask for more than He provides. If you do not take that first step of faith which is trusting in Him for your salvation, then you will lose everything on the final day.
"He also said, 'This is what the kingdom of God is like. A man scatters seed on the ground. Night and day, whether he sleeps or gets up, the seed sprouts and grows, though he does not know how. All by itself the soil produces grain - first the stalk, then the head, then the full kernel in the head. As soon as the grain is ripe, he puts the sickle to it, because the harvest has come.'" NIV translation
Jesus went on to show us our role in spreading the light of the gospel. He used the example of a seed that grows. The man only scattered the seed while God did everything else. It is the same with the gospel (seed) and us as we can only share the Word with people and it is only God that can use it to change a person's heart.
"Again he said, 'What shall we say the kingdom of God is like, or what parable shall we use to describe it? It is like a mustard seed, which is the smallest seed you plant in the ground. Yet when planted, it grows and becomes the largest of all garden plants, with such big branches that the birds of the air can perch in its shade.'" NIV translation
In the previous parable, Jesus made the point about the natural growth of a plant (and the kingdom) as being from God. The mustard seed when planted normally becomes a shrub but not a very large tree like Jesus described here. This is a picture of the unnatural growth of the church that has happened where many are perched in the building playing religion but not having a genuine faith in Jesus Christ. That is what He was talking about with the birds perching in the branches.
"With many similar parables Jesus spoke the word to them, as much as they could understand. He did not say anything to them without using a parable. But when he was alone with his own disciples, he explained everything." NIV translation
To the masses, Jesus told them about the things of God through these stories because there were only concerned with their physical state. But, to His disciples, He continued to explain their spiritual meaning as they were prepared to hear it.