Jesus does not explain this parable to us. Therefore, we should hold our opinions with a loose grip.
We do need to understand that the simple, little herb tree is sufficient in Jesus’ eyes for His redeemed. Therefore, it should be more than satisfactory for us.
How does God spread His kingdom? I don’t know. I just know that it is like the wind blowing and we can see and hear the effects (John 3). Those effects are because God has worked, and therein we should shout for joy!
When we read about the kingdom of heaven in the parables, or in other places, it important to understand that all the places being described are the dominion of God but may be located in different places.
The reign of Christ is right now not after a time of tribulation. Jesus Christ is on His throne at this very moment!