He put before them another parable: “The kingdom of heaven is like a mustard seed that someone took and sowed in his field; it is the smallest of seeds, but when it has grown it is the greatest of shrubs and becomes a tree, so that the birds of the air come and make nests in its branches.”
He told them another parable: “The kingdom of heaven is like yeast that a woman took and mixed in with three measure of flour until all of it was leavened.”
Jesus told the crowds all these things in parable; without parables he told them nothing. This was to fulfill what had been spoken through the prophet:
“I will open my mouth to speak in parables;
I will proclaim what has been hidden from the foundation of the world.”
These two parables, the mustard seed and the yeast, have the same basic message: Heaven is something small that grows into something much bigger. But what does that mean? How does Heaven grow? It could mean that Jesus, one person, is the seed, and as He spreads His message and more and more people follow Him the Kingdom grows. Or I guess it could have to do with the faith of those in the Kingdom. Your faith starts small and personal, but as our relationship with God grows it branches out and starts to reach other people through our speech and actions. Honestly I find these to be two of the hardest parables to understand, mostly because they are so short. Jesus doesn’t go in to detail, and the details are often what helps us figure out what parables mean.
Verse 34 goes on to say that Jesus spoke to the crowds only in parables. That means that there are some (probably many!) parables He spoke that weren’t written down. Too bad! But the words God wanted written got written. It also means that Jesus didn’t say anything to this crowd that wasn’t a parable. That must have been hard on them. Can you even imagine? He didn’t even explain the parables to the crowds, only to the disciples! And that fulfills yet another prophecy of Isaiah.