Why is everything on my laptop enlarged?

Usually when I plug my laptop in to my projector, things on the screen become larger to make them more visible from the projector screen. After I unplug my macbook from the projector, things go back to normal. But the last time I plugged it in, icons and images remained enlarged after unplugging. I have trouble viewing images and entire web pages because the computer displays everything so large and zoomed in. What can I do to bring my computer back to normal?