The keys to understanding parables fall along two important lines here.
First, all parables are meant to be simple expressions, taken at face value. We are always tempted to allegorize by attributing different identities and aspects to the elements in the story. I have done this repeatedly! Even the biblical editors did it, inserting Jesus explaining the parables. If Jesus had to explain his parable, then it was a lousy parable, like having to explain a joke - that's failure.
Second, we need to pay critical attention to the opening words: The Kingdom of heaven/God is like .... In other words, the parables is not some generalized information from which we can draw our own conclusions, perhaps by allegorizing the parable. Rather, it directly refers to the Kingdom and its radical nature.
The tiny seed and the pinch of leaven take on a whole new meaning. The incredible treasure in the field and the priceless pearl also take on new meaning as well.