The Kingdom of Heaven is real and powerful. It has always existed imo. It need not grow as Jesus said "it is in your midst".
John 16:13 But when he, the Spirit of truth, comes, he will guide you into all the truth. He will not speak on his own; he will speak only what he hears, and he will tell you what is yet to come.
John 3:5 Very truly I tell you, no one can enter the kingdom of God unless they are born of water and the Spirit.
It is not something growing around us. It is something to enter into.
God is holy and righteous. God's holiness and righteousness does not grow. God is complete and perfect. The Kingdom of God is a reflection of God's holiness.
God is ONE. The three lumps and the mustard plant are what people are making it. God is not making it three (like the lumps) or weak (like the mustard).
I meant it is only an idea to us right now. Of course it's real and it's right here among us, I believe. We can't see it- although we can feel it and experience it in our own way.