In all these chapters Krishna was giving details about how he has expressed in the world . But in the concluding verses he tells to Arjuna, that the whole world is infact a part of him which means it is an expression of him . So here he summarizes all that he had spoken in the earlier verses
While he tells that the whole world is a part of self , he makes a closer link between the creative process from where the world originates . He points out that it is the spirit which is the originator of the world . Sprit is full and whole , and the world is just a part of that spirit .
It seems to be very practical also . Whatever you are seeing in the outer world , that has the root inside . On the “inside”, which is meant by consciousness, is again root of all the expression which we see outside. World is a part of that consciousness which Lord Krishna is referring as his self . Consciousness is broad and wide and lot of it unexpressed or in the process of expression .
Why it is said that the world is a part of that expression ?
It is just because the entire Krishna conciousness ,could not be expressed in the world . It is just a part, which could be expressed and the rest is in the process of expression . So you see the world evolves , as more expression from the spirit starts adding to it .
So the evolution of the world has a direct relation with the expression of the spirit . World is not whole , but the spirit is whole and one . The world came out from the spirit, and it will be back to spirit . Once one understands it ,we becomes equanimous.