In contrast with dry skin, oily skin produces sebum, an oily or waxy substance that moisturizes and protects your skin and hair. The amount created may be so utterly high, it is uncontrollable and that can lead to pores clogging. This results in black heads, white heads, and ultimately enlarged pores.
When the excess oil is combined with bacterial contamination, the result is zits (or more technically: papules and pustules, depending on the external visibility of pus). The natural microbiome, beneficial microorganisms that serve as a physical barrier to prevent the invasion of pathogens, gets trapped in those clogged pores and the overgrowth may result in an infection, which manifests as a zit.