John 2:7

Fill the waterpots It is difficult to see the meaning of this command, if (as some contend) only the water which was drawn out was turned into wine. The pitchers had been partially emptied by the ceremonial ablutions of the company, i.e. pouring water over their hands. Note that in His miracles Christ does not create; He increases the quantity, or changes the quality of things already existing.

to the brim His Mother's words (John 2:5) have done their work. Our attention seems here to be called to the great quantity of water changed into wine.