Robert Frost, on Quora:
The ozone hole results from ozone depleting chemicals (e.g. chlorofluorocarbons (CFCs)). When CFCs are hit by ultraviolet light, they release chlorine. Chlorine reacts with ozone, destroying it to produce chlorine monoxide and diatomic oxygen.
These reactions are most effective when they occur on the surface of ice crystals, in the stratosphere. There are more ice crystals when it is most cold.
Antarctica is colder than the Arctic circle because the ratio of land to water is higher.
Therefore ozone depletion over the South Pole is greater than ozone depletion over the North Pole.