So, one answer to the observation "But wasn't it nice to have a way to express that concept?" is: not really, and anyone who wants one anyway has it at the ready. One can say "reduce by a tenth." Hopefully one will do so in comfort with the knowledge that reduce once had a different meaning—while comfort was once "to make strong," and strong once meant "narrow" [mine]!
- This is Auto-antonymy, correct?
No, because the antonym of "strong" is not "narrow", it is "weak", and the antonym of "narrow" is not "strong", it is "wide".
- Can someone please expound this auto-autonymy?
As in, what kind of a semantic change explains "narrow" > "strong"? Metonymy (in Traugott's sense): a sense which is contextually associated with the main sense takes over as the new meaning. The contextual lens applied to "narrow" is "constriction". If constriction results from deliberate activity (like say someone squeezing or pulling on an aperture to narrow it), then the constriction involves the exercise of strength: the stronger the agent doing the squeezing, the narrower the aperture. The deliberate activity of constriction by someone strong is then metaphorically extended to inanimate narrow apertures.
From https://en.wiktionary.org/wiki/strong, the transition happened in proto-Germanic: PIE *strengʰ- (“pulled tight, straight, constricted”), *strenk- (“taut, stiff, tight”) > Proto-Germanic *strangaz (“tight, strict, straight, strong”) (so *strangaz has been reconstructed as having both meanings).