What is the meaning of the parable of the mustard seed and the yeast?

I Was Asked;
What is the meaning of the parable of the mustard seed and the yeast?

These parables are a part of a much larger context which takes in several parables which you (as in so much of Scripture) have to take it all in to get the whole picture.

Matthew 13:24 Another parable put he forth unto them, saying, The kingdom of heaven is likened unto a man which sowed good seed in his field:
25 But while men slept, his enemy came and sowed tares among the wheat, and went his way.
26 But when the blade was sprung up, and brought forth fruit, then appeared the tares also.
27 So the servants of the householder came and said unto him, Sir, didst not thou sow good seed in thy field? from whence then hath it tares?
28 He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up?
29 But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them.
30 Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.
31 Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field:
32 Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof.

[Mark 4:32 But when it is sown, it groweth up, and becometh greater than all herbs, and shooteth out great branches; so that the fowls of the air may lodge under the shadow of it.
Matthew 13:3 And he spake many things unto them in parables, saying, Behold, a sower went forth to sow;
4 And when he sowed, some seeds fell by the way side, and the fowls came and devoured them up:]

Matthew 13:33 Another parable spake he unto them; The kingdom of heaven is like unto leaven, which a woman took, and hid in three measures of meal, till the whole was leavened.

[Matthew 16:11 How is it that ye do not understand that I spake it not to you concerning bread, that ye should beware of the leaven of the Pharisees and of the Sadducees?
12 Then understood they how that he bade them not beware of the leaven of bread, but of the doctrine of the Pharisees and of the Sadducees.

The "leaven" is false doctrine that will eventually corrupt the Church.]

34 All these things spake Jesus unto the multitude in parables; and without a parable spake he not unto them: . . .

47 Again, the kingdom of heaven is like unto a net, that was cast into the sea, and gathered of every kind:
48 Which, when it was full, they drew to shore, and sat down, and gathered the good into vessels, but cast the bad away.
49 So shall it be at the end of the world: the angels shall come forth, and sever the wicked from among the just,
50 And shall cast them into the furnace of fire: there shall be wailing and gnashing of teeth.

The point is, we often hear how the Church is going to grow and get better and more spiritual and eventually be prepared to meet the Lord.

What we do not hear so often is that as time goes on the Church will become more and more self-serving and the Word of God will be the last thing on its mind. We will make it up as we go along and adopt more and more false doctrine and the organizational churches will eventually become a habitation of devils. They will lodge in the very branches of the Church and the “tares” will be indistinguishable from the wheat. "Are we there yet?"

Of course, we see from the context that the Wheat and the Tares make up the great Mustard Tree or the three measures of meal. They are the same and become corrupt. Filled with tares, Leaven, or the demons. All amount to the same thing and point to general apostasy of the Church. But God always has His Remnant and He will gather them out and put them into His Barn.

Matthew 13:30 “Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.”