The mustard seed parable is found in three of the gospels.
Matt 13:31-32 (KJV)
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: 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 4:31-32 (KJV)
[It is] like a grain of mustard seed, which, when it is sown in the earth, is less than all the seeds that be in the earth: 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.
Luke 13:19 (KJV)
It is like a grain of mustard seed, which a man took, and cast into his garden; and it grew, and waxed a great tree; and the fowls of the air lodged in the branches of it
KJV does the best translation out of all the translations.
A lot of other translations say the mustard seed is smaller than all other seeds. Which is not true. A mustard seed is typically around 1/20th of an inch. Whereas a begonia seed is 1/100th of an inch. And a mushroom seed is microscopic.
In the KJV, the mustard seed is described as “least” not “smallest”. In Greek, it can mean least in terms of size, space, age, time, quantity, rank, or influence. So Jesus was not saying it was the smallest of all plant seeds, though it was small. What Jesus said was that it was the least.
least (3398 mikros) small, little (of size, space, age, time, quantity, rank, influence)
Mustard trees (more properly called shrubs) do not grow tall enough for birds to make nests in. They could possibly grow to 15 feet in height. But many translations say birds could “nest” in them. In Greek, one meaning of lodge is to rest. So, the meaning makes more sense that birds could rest in mustard trees rather than live in them.
lodge (2681 kataskenoo) lodge, rest
There are multiple interpretations of this parable (which I’ve linked some below). But the basic concept that all interpretations agree on is that Jesus is presenting something really small growing into something big. And showing that God’s kingdom grows the same way. It may seem small at first, but it’ll grow into something large.
Interpretations: