I upgraded Lubuntu on my laptop to 16.04 and added some icons. No matter what icon set I choose, whether it be one I have added or one of the default icon sets, some windows and the login screen have large buttons in the top panel. I even opened up the index.theme file of the icon set and changed MinSize for each of the scalable icons to 8. That didn't work. I've attached a couple of screenshots to show you. Again, I've tried different icon sets and still get the same effect. Even some of the icons in the drop down menus are large.