Why does the size of the atom increase down the group?

Why does the size of the atom increase down the group?
Answer:
In moving down a group, the charge on the nucleus increases with increase in atomic number, but at the same time, there is an increase in the energy levels. The number of electrons in the outermost shell, however, remains the same. Since the effect of additional energy levels outweighs the effect of increased nuclear charge and thus the distance of the outermost electron from the nucleus increases on going down the group.