A norma in Latin is a carpenter's or mason's square, so useful for making sure that the corner cupboard fits the corner, and enormous was a good 16th-century word for something that was out of line, or didn't accord with the usual state of affairs, just as the word egregious (derived from grex, a flock) meant outside the common herd.
It was not at first thought that there was anything wrong in being egregious - kings and princes were indeed sometimes so called - and it was only later that people began to see egregiousness as something beyond the pale. Nor was there any shame attached to enormity. Perhaps it was the English character, and the belief that one of the duties of a gentleman was to be inconspicuous at all times, that began to give the word a bad name, so that it wasn't long before enormity was attracting adjectives such as "gross", "criminal" and "wicked".
Lord Mayhew himself is, of course, a perfect gent. There is also something pleasantly old-fashioned about him, so I suppose one shouldn't be surprised at finding him using the word enormity in its pristine sense, and what has happened in Belfast is indeed a prodigious event.
He was certainly not committing the fairly common solecism, as I was always taught to call it, that enormity has something to do with bigness. The 1989 Oxford English Dictionary marks that meaning "obsolete" and its editors couldn't find any modern examples so they had to make one up themselves, which I always think is a bit of a cop-out, though other dictionaries make quotes up as a matter of course. But this is a nice one: "'You have no idea of the enormity of my business transactions', said an eminent Stock Exchange speculator to a friend. He was perhaps nearer the mark than he intended." Collins labels enormity=large as "informal" and the Encarta World Dictionary, giving "great significance" as one of its definitions (which is what Lord Mayhew really meant), adds a helpful note recommending the word immensity.
But why did enormous, having at first meant "unusual" and then "shocking", go on to mean "particularly large"? (One might equally ask why vast and vastly went in the opposite direction, starting out as words for "wide" and "widely" before coming to mean no more than "exceptional" and "very".) Elizabethans who spoke of a man having an enormous appetite weren't necessarily commenting on how much he managed to get down; he probably just liked eating mice, hedgehogs and suchlike "small deer". I'm not sure how the change happened, except for the fairly obvious connection between outlandishness and excessive bulk; monstrous and prodigious, originally reserved for the alarming or the miraculous, have strayed from their early meanings in much the same way.