Thanks for your comments, stratotom. As to my second problem, here are two large images (in case you don't have any lying around your hard drive):
(Option-click them to download. They're Nikon D700 sample images, if you care.
Open the first in Preview. Note that zoom works as it should. Open the second one. Doesn't the zoom button now fail to maximise the window? In my case, at least, it leaves the second Preview window smaller than my screen.