Why would the display be larger than the screen?

My Mac has started opening with a screen that is larger than the display. (I may not have the terminology quite right) Here's a non-techie description...if the phyical display is 1680x1050, Finder or Safari open up at, say, 1720x1200. If I want to have the toolbar visible at the top, I cannot see the dock at the bottom. If I want the Apple to be at the top left corner, I cannot see the time at the top right. In both cases it is one or the other. Restarting has no effect on this annoyance.