The issue here is that "enormous" is an adjective, and in those all-too-frequent cases in which a noun is called for, lazy writers cheaply noun-ify the adjective. This practice is even odiouser than lazily adverbifying an adjective. (See what I mean?)
The problem is that "enormity" already has a well-established definition. An "enormity" is a monstrous injustice, a grave moral wickedness, an ugly sort of travesty. Think of it this way:
- Hurricane Katrina was enormous.
- The aftermath of Katrina was widely regarded as an enormity.
Be not deceived, dear reader: the definition matters. These two possible choices of definition for "enormity" are nearly impossible to differentiate based on context alone. Consider the following: "She was stunned by the sheer enormity of the thing." Two quite different meanings arise, do they not?
(For the inevitable exception to the rule, I suggest the following: The healthcailoutulus bill was an enormity. Works on multiple levels, that.)