Screen size increasing

I have a late 2012 IMac and am doing some unknown thing with the mouse that makes the print size suddenly increase on my screen. I don't know what I am doing nor do I know how to reduce the size after it happens.
Thanks for any help.

Check your "Universal Access" or "Accessibility" settings in System Preferences.

- Nick

You're probably zooming in on the picture. What kind of mouse are you using?

If you are using an Apple Magic Mouse, go here and also watch the videos to learn gestures.

I know exactly what you are doing since I do it myself. You are absent-mindedly tapping (not clicking) twice on the Magic Mouse, which invokes a zoom. Try it deliberately and you'll see what I mean. Do it again to zoom back out.

You can turn this feature off in your system preferences, but once you know what is causing it, you can avoid doing it inadvertently and I've found it's actually quite a handy little feature.