You Can Grow Faster
Here are two similar parables placed together for emphasis. Jesus starts both stories by using a simile: "The kingdom of heaven is like..." In each case the point is that small beginnings can produce great results.
I am sure those original disciples had no idea of the worldwide and historic nature of their work. They could only see the present, not the future. Jesus knew that, although their labor seemed almost insignificant in the wide world, it would result in a great harvest.
That is the way the kingdom works. Followers of Jesus don't focus on capturing kingdoms as much as touching individual hearts and lives. This tiny seed, this small invasion in the principalities and powers, works mysteriously, behind the scenes, until its growing and gigantic influence is seen by all.