Let Those Windows Breathe!
Since version 3.0, Photoshop has done something called “protecting the palettes” (I don’t know if that’s its official name, but we’ve always heard it called that). What it means is that as you increase the size of your image using the Zoom tool (Z), Photoshop stops increasing the size of the image window when it reaches the left edge of your open palettes (if you have turned on the Resize Windows To Fit checkbox in the Options Bar). When it reaches this safety zone, the window stops growing, and only the image within the window continues to zoom. The only way to get around this (in previous versions of Photoshop) was to close your palettes. Then you could zoom the window as large as you’d like. However, Adobe addressed this problem back in Photoshop 6, and now if you want to keep the window growing, choose Ignore Palettes in the Options Bar when using the Zoom tool.