TOK theme use 
statusbar label,
statusbar frame {
    color: @theme_fg_color;
    font-size: smaller;
}

for statusbars in gtk3.
And the theme is optimize for 10px font-size.
You can edit /usr/share/themes/TraditionalOk/gtk-3.0/gtk-widgets.css
Use 'font-size: normal' at line 196.

First of all thank you very much for your input!

In order to have the modification survive package-updates I have copied the whole TraditionalOk folder to .local/share/ and edited the gtk-3.0/gtk-widgets.css there.

Do you know if there's a way that I can avoid a full copy and just keep my deltas in .local/share/?

see https://github.com/mate-desktop/mate-panel/issues/507#issuecomment-267675457
Here using  ~/.config/gtk-3.0/gtk.css is advised.
But i tried to generate this file with

statusbar label,
statusbar frame {
    color: @theme_fg_color;
    font-size: normal;
}

before i answered to you.

it doesn't work for me either.
Oh well, thanks anyway. :)

Hi Wolfgang,

I found why it didn't work: "normal" is not a valid value for "font-size:".
In fact GTK issues a warning while parsing it:
Gtk-WARNING **: Theme parsing error: gtk-widgets.css:196:15: not a number

So I created ~/.config/gtk-3.0/gtk.css with a valid value and it works:

statusbar label,
statusbar frame {
    color: @theme_fg_color;
    font-size: 90%;
}

Great, nice finder :-)
...i guess you know now to hack a theme for personal usage.
gtk3-inspector is the tool to find any selector for a gtk3 application, except firefox and libreoffice.......