Wellll, Earth Oliver, sort of right: I changed it back to 100%. But since I never changed the setting in the first place, the mystery is still there.
After a day and a half with Kelby Tech, and desperate to overturn everylastrock before I would be forced into a probable entire uninstall/reinstall (and years' worth of personal settings!), I clicked on "this blue link thingie under the ACR image." I'd never done it before (no need).
And then, I saw a box checked that said the file size was limited to 5MB! Whaaat? The dropdown gave another choice of full size. Voila! That was it! So %&@$!! simple!
So, yes, it's "fixed." Thank God! But still no answer on how it was changed in the first place. Oh well, Tech kept saying how "buggy" PS could be. I'd just never experienced it before.