Maximizing iTunes
When clicking the + button in iTunes, the view will switch to the “Mini Player.” It is possible to change the behavior of the “+” button to maximize the window.To switch to the Mini Player, you could still use the keyboard shortcut Command+Shift+M.
If you would prefer to maximize iTunes, rather than switch to the Mini Player, user the following command in Terminal:
defaults write com.apple.iTunes zoom-to-window -bool YES
If you want to return the settings to the factory default, enter the same command but change YES to NO.