It is possible to decrease the size of toolbar by adding (or editing) the following line in your /etc/gtk-3.0/settings.ini file to make your icons small:
gtk-icon-sizes = panel-menu=16,16:gtk-large-toolbar=16,16
You will then notice the différence in GTK application like gedit.
You can also use this command to change the toolbar size of all gnome application:
gconftool-2 --set --type=string /desktop/gnome/interface/toolbar_icons_size "small-toolbar"
and this command to restore the default large icons
gconftool-2 --set --type=string /desktop/gnome/interface/toolbar_icons_size "large-toolbar"