As you can see in the screenshot the top menu bar is overly large. Firefox by default has similar behaviour but is able to shrink with https://addons.mozilla.org/EN-us/firefox/addon/htitle/
Happens on Ubuntu Gnome 15.04 x64
Looks like your theme’s doing something weird. How come you’re using Gnome? 
I think that’s just the default way the Adwaita theme displays applications with menus. Is this any different than Gimp, for instance?
Apps not written specifically for Gnome 3 aren’t going to use its headerbars. Firefox has that theme because it’s extensible and a user wrote it.
You could always tweak your Adwaita theme to show smaller titlebars, I suppose. I think the CSS for Gnome3 style apps and default menu-style apps is separate in the theme. But that would be up to the theme author, not the Unity devs.