22 For a diocese to fulfill its purpose the nature of the Church must be clearly evident to the people of God who constitute that diocese. To this end also bishops must be able to carry out their pastoral duties effectively among their people. Finally, the welfare of the people of God must be served as perfectly as possible. All this demands, then, a proper determination of the boundaries of dioceses and a distribution of clergy and resources that is reasonable and in keeping with the needs of the apostolate. All these things will benefit not only the clergy and Christian people involved, but also the entire Catholic Church. Concerning diocesan boundaries, therefore, this sacred synod decrees that, to the extent required by the good of souls, a fitting revision of diocesan boundaries be undertaken prudently and as soon as possible. This can be done by dividing dismembering or uniting them, or by changing their boundaries, or by determining a better place for the episcopal see or, finally, especially in the case of dioceses having larger cities, by providing them with a new internal organization.
Source: Christus Dominus (Vatican.va)