Reduce title bar size for applications in Gnome by creating ~/.config/gtk-3.0/gtk.css with the following contents

gtk.css

# Gnome 3 - based on https://blog.samalik.com/make-your-gnome-title-bars-smaller/
.header-bar.default-decoration {
    padding-top: 3px;
    padding-bottom: 3px;
    font-size: 0.8em;
}
.header-bar.default-decoration .button.titlebutton {
    padding: 0px;
}


# --------------------------------------------------


# Gnome 3.20+ - based on https://blog.samalik.com/make-your-gnome-title-bar-smaller-fedora-24-update/
window.ssd headerbar.titlebar {
    padding-top: 4px;
    padding-bottom: 4px;
    min-height: 0;
}
window.ssd headerbar.titlebar button.titlebutton {
    padding: 0px;
    min-height: 0;
    min-width: 0;
}