Unwanted 'smart zoom' when using Wacom tablet

I'm using a Wacom tablet rather than a mouse. If I move my stylus across the tablet, sometimes that results in an unwanted "zoom-in" on the page. I have no idea how to zoom out, which means I have to close the page or document and reopen it. Stupid waste of time. Any solutions?

[Re-Titled by Moderator]

Original Title: Unwanted, unexpected "smart zoom"