Well, no worries, you don't have to edit any files (the hard way) and our extremely creative artist called half-leaf has done just that for all of us who are annoyed by this "Adwaita" GTK3 theme.
Not only he has reduced the size of those toolbars but other improvements such as blending the sidebar with the theme in General, reducing scroll-bar size and enhanced button padding are also "fixed". All you gotta do is first download the zipped package from here. Then extract the content and the copy that extracted folder to your "./themes" folder in the "Home" of your GNU/Linux PC/Laptop.
Yep... that's pretty much it... again, thank you half-leaf, for this :).