If you maximize the game window at/ before the title screen, when you open a world, the game view is the same size as the original windows, before being maximized. If you resize the game window, it fixes it.
Ps. For Mac OS X, you should add native fullscreen support (works with mission control)
Duplicate of MC-13661, please use the search function to see if your bug has already been submitted. Currently over 52% of tickets are being closed as duplicate.