Skin pores provide skin oils to keep skin soft and keep skin hydrated. When pores are dilated, infected, or clogged, pores appear bigger than they actually are. They become "large pores".
There are some ways to make large pores look smaller, but it is difficult to permanently shrink large pores.
There are two major factors that cause large pores: genetic and aging.
Oily skin tend to have large pores. Aging process, sun damage, will also cause pores to dilate. When dirt and bacteria clog the pores, skin oils begin to accumulate, causing the pores to expand.