The logical thing is to avoid using either word (decimate or enormity) so as to communicate clearly and avoid hard feelings.

I do the same with "comprise." I avoid it.

(Come to think of it, I wonder if they guys in the togas ever actually managed to decimate a legion. Can we suppose that the big, brutal, barbaric legionnaires simply lined up to be counted and killed? I suspect this is another one of those bad things in history which never actually happened.)