The mustard seed and the leaven seem to go together in terms of the point Jesus was making.
Thinking back, I think I have mostly heard these parables preached in terms of great growth from small beginnings, and that certainly is an aspect of the word pictures Jesus used. Small seed, large plant. Small amount of leaven, large amount of dough.
But it seems to me there is another aspect of these two parables we might miss. In both parables, the great action happens behind the scenes. The seed is planted. The leaven is hidden. Great big things happen.
The kingdom of heaven is not something that we are doing. It is God’s work. It is happening mostly outside of our viewport. When it is fully complete, it will surprise us with its size and scope.
The plant is big enough for nesting and the dough can feed a large crowd. Both started humbly. But the man had nothing to do with the growth of the plant and the woman had nothing to do with the action of the yeast. Both counted on the established ways of using them.
We must trust God for the kingdom. Sometimes it is discouraging to be involved in it and there is much, much waiting. But God is working.
Jesus spoke in parables. The parables are like seeds and yeast. We need to let God grow them in us.