【Enormity】 , 【enormousness】  both mean the state or the quality of being enormous but are rarely interchangeable in modern usage.

【Enormity】 imputes an abnormal quality; it applies especially to the state of exceeding all bounds in wickedness or evil, and therefore of being abnormally, monstrously, or outrageously evil.

【Enormousness】 applies to the state or quality of grossly exceeding comparable things in size or amount.

【Enormity】 , but not  【enormousness】 , may also be used of an instance of what is characterized by 【enormity】 or monstrous wickedness.