Reduce icon size in panel without reducing panel height

As you can see in the attached image below, the two icons (Sound and Notification Manager) on the right side of the panel are too large to my liking. I think it is because I increased the panel height itself and the size of the icons depends on that height. However, I still prefer the said icons to have a reduced size similar to when the panel height is reduced but I would not want to reduce the panel height any more. Is it possible to do so?

I tried modifying my ~/.config/gtk-3.0/gtk.css file by adding the following lines:

#PanelApplet box.horizontal {
  padding-top: 8px;
  padding-bottom: 8px;

but all this does is crop the icon. Moreover, this only gets applied to the Sound icon.