0000000000000000000000000000000The dimensions of the Stage (and the parent Group containing the Display Objects) is equal to the absolute value of the combined dimensions and positions of every Display Object in the Group, on the Stage, and in the World.
Example: if you have a World that is 100 (width) x 100 (height) but bounded to -50, -50, 50, 50 such that 0,0 is in the center of the World, and you have 2 display objects that both have the same dimensions of 10 (width) x 10 (height), then the width and height of the Stage will vary based on their placement within the world.
If the first display Object is at xy location (-50, -50), and the second display object is at xy location (40, 40), then the dimensions of the Stage and the parent Group will be 100 x 100.
However, if they are placed at (-10, 0) and (0,0) respectively, than the dimensions of the Stage and the parent Group will be 20 (width) x 10 (height).
As of Phaser 2.2.2, the camera implementation appears to move the World coordinates only; zooming / panning must be poly-filled using Group scaling and adjusting the pivot property of the parent Group.
EDIT: Zooming is now working in later versions of Phaser.