To pick on Ann Patchett a bit more (see “Only the Lonely”) consider this sentence: “Marina hadn’t understood the enormity of the river until she was on it.” And somewhat later in the book: “The enormity of Marina’s happiness was caught in that light.”
Garner’s Modern American Usage (Third Edition, Oxford 2009) says, “The historical differentiation between these words should not be muddled. Enormousness = hugeness, vastness. Enormity = outrageousness, ghastliness, hideousness…. The OED notes that ‘recent examples [of enormity for enormousness] might perh. be found, but the use is now regarded as incorrect.”