Usage Note: Enormity is frequently used to refer simply to the property of being great in size or extent, but many would prefer that enormousness (or a synonym such as immensity) be used for this general sense and that enormity be limited to situations that demand a negative moral judgment, as in Not until the war ended and journalists were able to enter Cambodia did the world really become aware of the enormity of Pol Pot's oppression. Fifty-nine percent of the Usage Panel rejects the use of enormity as a synonym for immensity in the sentence At that point the engineers sat down to design an entirely new viaduct, apparently undaunted by the enormity of their task.
This distinction between enormity and enormousness has not always existed historically, but nowadays many observe it. Writers who ignore the distinction, as in the enormity of the President's election victory or the enormity of her inheritance, may find that their words have cast unintended aspersions or evoked unexpected laughter.
From Dictionary.com
This distinction between enormity and enormousness has not always existed historically, but nowadays many observe it. Wussie.
Checking Oxford, it seems that the words "enorm" and "enormity" entered English (from the French énorme and émormitié) in the late 15th Century. The meaning being, as might be expected: "out of the normal, abnormal".
They were joined by "enormous" (from the Latin enormis) c.1531.
The first noted use of "enormity" with the specific meaning of "menstrous wickeness" is dated at 1538.
The first noted use of "enormous" with the specific meaning of "extraordinary in size or quality" is dated at 1544.
It seems to me the distinction was developing from the start. It ill-behoves us to go against the tradition of half a millenia.