In the verse above, the term fullness does not necessarily mean quantity because no one can measure the vastness of God who is Spirit for He fills heaven and earth. It refers more to quality, meaning, all the fullness in quality is in Christ.
In analogy, when you get a sample of saltwater from a vast ocean and place it in a container. If you examine the quality of the ocean in that container, the properties you will find is the same exact properties as of that vast ocean. In this sense, all the fullness in quality is in that container of that ocean.
The container is typology to the Son either in glorified form or in human form and the Ocean is THE ETERNAL LIFE (John 17:3) who is in all that has life (Ephesians 4:6) but, the exact quality is in the Son only or the Image of God hence begotten Son.
For this reason, only the Son can represent the fullness quality of THE ETERNAL LIFE, no one else. He is the model which we should at our best try to imitate.