Photoshop for Mac, both CS4 and the latest CS5 editions, have a quite peculiar default option enabled.
While zooming in or out of any opened image, the image window is being automatically resized.
Personally I find it on one hand a bit helpful for the zoom-in function, but on the other hand highly annoying for the zoom-out function.
If you are trying to disable this feature, then simple go to:
Photoshop -> Preferences -> General…
And uncheck the option “Zoom Resizes Windows”, as shown on the screenshot below.
On a side note, this option is not enabled by default on the Windows Photoshop version.