Doubts are all round my head. I'm strongly leaned toward an operating-system-wide trouble since more than one software I tested is affected by bigger fonts: browser and Gimp.
I tried to shrink the set of possible causes: since I run a custom back-port of kde3 that doesn't seem to have any control panel for fonts configuration, my only hook was to manipulate those X files in /etc/X11/Xresources/ and append to them a config line of the type:
and after altering that 96.0 to other values, I started to notice font changes in all the desktop.
The Xft should be the font server for GTK-based applications; the role it showed to play was to enlarge or shrink the font sizes of all software's window menus and buttons, that is, all what structures the upper part of program's windows, but it had no effect at all on the texts contained inside those windows, so that browser (and Gimp) showed exactly the same result.
This should mean it is not a GTK-rendering problem.
Given that was my only hook, I installed the XFCE4 desktop environment aiming to access its fonts control-panel. But then, it had the same exact effects as modifying the earlier Xresources files; This should mean xfce4 control panel acts on fonts thru GTK's render, rather than on some other render.
Other renders still to test are the X core one and the Java one. These at least are all what I know about font rendering. I have no idea however if Iceweasel (Firefox) or Gimp actually use those system renders or incorporate one of their own.