A little leaven leavens the whole lump.
Arthur Pink has an interesting book on the parables of Matthew.. He points out that in the time of Jesus leaven was a symbol of sin.. and so this parable is not about "church growth" but about evil entering the church and eventually taking it over
looking at the "mega church's" and the Joel Oilseed's, Joyce Meyers etc... we may be seeing the fulfillment of that parable