Well! I almost killed this after posting as a late night rant!
think about how decimate would have been used in (A) and (B) and how the meanings would have become blurred. 1856 J. H. Newman Callista The population is prostrated by+pestilence, and by the decimation which their riot brought upon them. what percentage do you suppose was meant here? we can't know, out of context, but it was certainly not 10%.
[ref: red] But that's exactly my point. I *don't* know what percentage might be meant, only that it is significantly greater than 10%. Since the change is only "very large," and the result of a specific multiple, the impact on natural language is much stronger. The extrapolation need not be complete or precise -- only extreme. It this case, it is the sense of the extreme, not its quantity, that is expressed. It seems to me this comports well with the late 17C meaning you cite, "(A)the killing or destruction of one in ten and (B) destruction of a large proportion; subjection to severe loss, slaughter, or mortality."
(Note: "extrapolation" is probably an unfortunate choice of words, it was the best I could come up with.)
tsuwm, do you have anything on the orthographical shift phenomenon.
