My situation and my question are like the ones in http://lists.gnu.org/archive/html/help-gnu-emacs/2012-06/msg00183.html:

after the recent upgrade to emacs 24.x (...) my font size is too large (one times C-x C-minus corrects it temporarily for the buffer). The menu font size is fine, the rest (buffer, mode line, minibuffer is too large). I've read about the various ways to set the font size. Is there a specific reason why the font size has increased after the update? And thus a global way to correct it?

So, even having read about all the ways to set a font in Emacs, I haven't found an answer which would explain the difference in font size between an uncustomized Emacs 23.x and Emacs 24.y.

Probably, it's as in that answer:

Emacs 24 adds support for gconf and gsettings, so it'll pick whatever monospace font and sizes you have set as the default in your system settings

I'd like to know the place these settings are stored, and to tweak them (so that not only Emacs, but perhaps other similarly behaving programs would get my font settings).

I'm especially interested in making the default font size smaller again.

(Now, I'm using Xfce.)