First of all, if an imaginary line is drawn through the center of the hydria, you can see that the two halves are mirror images of each other. This means it is SYMMETRICAL, and symmetry helps to give balance to the hydria.

The size of the painted images is in PROPORTION to the part of the hydria they are painted on. The largest image is on the largest part, and so on. This contributes to the balance and harmony of the hydria.