The parable of the mustard seed, like any parable can have multiple meanings. A story allows each hearer to find something meaningful, even if those meaningful somethings are not alike from one person to the next. Some night see this parable as the Christian Church starting from one person but producing a worldwide organization that affects the whole world. But the same is true of every human organization.
The imagery of a large tree that allowed the birds of the air to come and make nests in its branches was earlier use in the book of Daniel. It was a dream that the Babylonian King Nebuchadnezzar had and which Daniel interpreted. The tree in this dream signified the greatness of the empire Nebuchadnezzar had built and how under the influence of that empire other peoples found food and security. Everyone benefited.
This would be common knowledge for the original hearers of this parable. I think it was meant to talk about the sphere of influence we each can have in our fields. It speaks to what happens when we tap into our spiritual side. When the power of our spirit is allowed to grow it becomes great. It effects much more than simply what is within us. Those around us are blessed.
When one young man came out as gay in college he chose three people to tell. Why did he choose those particular people? Their true spirituality have him the sense that he would be safe with them. What a compliment to those people.
We would do well to think less about the size of the plant that has grown from the mustard seed and more about whether others consider our hearts a safe place to be.