Could it be autohinting? It makes certain fonts (or perhaps any font when in bold) wide and blobby. KDE doesn't offer to enable this.
That would enable it for all users. If you want to make it user-specific, add this tidbit to your ~/.fonts.conf file.
$ sudo ln -s /etc/fonts/conf.avail/10-autohint.conf /etc/fonts/conf.d/10-autohint.conf
Omit the parts in red if they already exist.
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<edit mode="assign" name="autohint">
<bool>true</bool> <!-- autohinting -->
The changes should affect newly started applications. If you start them from a terminal, it will output errors if your .fonts.conf file has errors in it (with the line number where the error is), so that might be a good idea, to make sure it's set up properly.