nOw2 wrote:
So, my question is : is there a linux flavour that processes the package installation in a efficient way, installing only the common files among the packages instead of copying them all ?
No, install the files by hand from .tar.gz
More seriously, tracking the dependencies to this level of 'granularity' would get insane, and create the corrolary - "I already installed B, why is this file missing?!"
The best way to look at it is to consider KDE+Qt or Gnome+GTK as Windows and Geoworks were atop MS-DOS. Both 'very large' packages, and, had you the disk space, you could've had both installed at once. (From that perspective, we've 'advanced' in the sense that you can run Qt apps under Gnome or GTK under KDE, because the apps just link to their desired widget lib and X11 doesn't actually care.)
Now, if you want the whole 'user experience,' you'll have to install one or the other - once - at the cost of a fair bit of disk. (You think the binaries are bad? I run FreeBSD, people think I should build from source!

)
If you want to save disk space, you'll want to limit yourself to apps that, at worst, link to GTK or Qt (and preferrably only one), without depending on *any* of the Gnome or KDE libs. (My opinion is that, in practice, there are more 'decent' apps that use GTK sans Gnome than use Qt sans KDE; YMMV.)
Debian is supposedly good about following dependency trees, but they made the mistake of putting.. dselect, is it?.. at the end of the installer script, and presumably dselect is dumber than actual apt. Maybe. There might also be options (a-la BSD Ports make directives) to say 'don't install 20MB of .ps documentation with this one;' you'll have to ask someone familiar with your distro's idea of packaging.
I'd guess 20GB is now the baseline 'enough' to install all of an X11 server du jour, Mozilla, Gnome, KDE, and whatever passes for other reasonable software while still leaving most of the disk free for some data.

[My FreeBSD install, with two versions of Phoenix/Firefox, Linux and BSD builds of Java, GTK and Qt but none of the 'big' desktop environment libs themselves, and assorted other cruft, is probably pushing 7GB now, but I haven't needed to install something in an age.]