It seems to me that the Author intends us to read the parable of the mustard seed and the parable of the leaven together, hence the periscope of v31-33:
31 Another parable He put forth to them, saying: "The kingdom of
heaven is like a mustard seed, which a man took and sowed in his
field, 32 "which indeed is the least of all the seeds; but when it is
grown it is greater than the herbs and becomes a tree, so that the
birds of the air come and nest in its branches." 33 Another parable
He spoke to them: "The kingdom of heaven is like leaven, which a woman
took and hid in three measures of meal till it was all leavened." (Mat
The NAC commentary notes:
Jesus’ next two parables prove closely parallel. They are not
full-fledged narratives but short analogies or similes. Each presents
only one main character and probably teaches only one central truth.
Jesus likens the kingdom to a mustard seed or lump of yeast that grows
from inauspicious, seemingly insignificant beginnings to attain a
greater size (the mustard seed) or have more widespread influence (the
leaven) than many would ever have suspected. The current manifestation
of God’s reign within Jesus’ small band of disciples seems relatively
impotent; one day many will be astonished about how their movement
grew and impacted the world.1
In the book He Spoke in Parables, Keddie identifies the mustard plant in question as most likely being the black mustard (brassica nigra) which is an annual, belong to the cabbage family. This plant grows rapidly and can apparently attain a height of 3 metres in the right climate, yet it comes from a tiny seed - the point seems to be that from inauspicious beginnings the kingdom of God will grow tremendously in the growing season.
In regards to following (linked) parable of the leaven, the women in question is making a large batch of bread. "The “measure” here indicated, though not always and everywhere identical in capacity, is generally held to have averaged about a peck and a half. Three such measures would therefore amount to a huge quantity, not less than an ephah; one might say “more than a bushel.” But it was not at all unusual for a woman to make so large a batch. Sarah did it (Gen. 18:6). A similar amount is also mentioned in Judg. 6:19 and in 1 Sam. 1:24."2
Again, the point is the same, it doesn't take much yeast to provide a massive batch of dough, once the yeast is added (and fed) the fermentation process will continue until the whole batch has risen.
Some commentators (eg. Hendriksen) suggest that the parable of the leaven refers to the inward growth of the kingdom and the the parable of the mustard seed to the outward growth of the kingdom - which ever opinion we take on that the basic point is that the Kingdom of heaven is going to grow beyond what anyone would humanly expect.
1 Blomberg, C. (1992). Matthew (Vol. 22, pp. 219–220). Nashville: Broadman & Holman Publishers.
2 Hendriksen, W., & Kistemaker, S. J. (1953–2001). Exposition of the Gospel According to Matthew (Vol. 9, p. 567). Grand Rapids: Baker Book House.