My issue is solved. Found the culprit. I already mentioned that the built in window styles are buggy in resetting and ignoring icon sizes and that somehow my window style always gets reset to the built in cleanlooks style. The question was just why that style as I never had used it wittingly.
After the confirmation here in the forum that no-one else has the problem with the reset style and therefore icon sizes I supposed that the installation of RazorQt had caused the trouble (freundliches "Klappe, agaida..."
). RazorQt isn't installed on my system anymore but has presumably left some marks. Apparently RazorQt writes a [Qt] section with some default configuration into the file ~/.config/Trolltech.conf, i.a. "style=Cleanlooks". I have verified that it's RazorQt by looking at the file in a Firestarter RazorQt image in virtualbox. And my fresh Firestarter KDE install in virtualbox doesn't have that entry.
KDE SC seems to use the [Qt] section too. Alas it doesn't write the window style into it - but reads it from there! And obviously gives it - since KDE SC 4.11.2 - a higher priority than the settings the user made in systemsettings.
So after backing up the Trolltech.conf file I have deleted the whole [Qt] part from it and instantly, without re-login, my systemsettings' icons were normal again. Confirmed my "nameless" style of choice in systemsettings and now everything is ok.
Just for your info...
What remains are the buggy built in window styles but that doesn't bother me as I don't use them (wrote bug report on bugs.kde.org).