Here's my 2 cents worth. With icon text turned on the smallest the window would "shrink to" is the icon/text bar.
I turned the icon text off and the icons were much smaller and lot's less space between them. The main window now seems to NOT be limited to icons but something else.
Here is a snapshot, notice lot's of room left on icon bar but this is the smallest my window will go. (Which is fine! I operate full screen anyway.)
I love the new "Mac standard" look btw!
(ps: I'm completely redoing my library, thus, only 4 books in the lib!)