Two significant factors that influence the size of the ozone hole over the southern pole are temperature and the polar vortex.
A number of sources confirm that the ozone hole increases in size when the temperature over Antarctica is colder. (Parrondo et al, The Ozone Hole, Sam Houston State University),
The polar vortices that form over the northern and southern poles are different because the northern vortex forms over water that is surrounded by land, whereas in the southern hemisphere the situation the opposite; the vortex forms over land surrounded by water. Because of this, Antarctica "is meteorologically isolated from air at higher latitudes during winter".
This physical condition forms an isolated air mass swirling over Antarctica called the south polar vortex. In the air of the polar vortex, the temperatures drop to very low levels in the stratosphere, below -80 degrees Celsius. At these temperatures, chemicals present in the stratosphere freeze out and form polar stratospheric clouds (PSCs). It is the chemical reactions that occur on the PSCs that result in the large decrease in ozone during each austral spring over Antarctica that we called the Antarctic ozone hole.
Chlorine and fluorine oxides, produced by stratospheric photolysis (sunlight induced decomposition) of Chlorofluorocarbons (CFCs) also contribute to the depletion of ozone. During the southern winter, Antarctica is predominately dark due to the lack of sunshine. With no sun light photolysis of CFCs does not occur. During the southern summer when Antarctica is continuously sunlit CFCs decompose, further reducing ozone levels within the polar vortex.