If you are pressing the "Control" button on the Mac keyboard, scrolling up will zoom in and scrolling down will zoom out. You can disable this if it annoys you through System Preferences > Universal Access. If you are using Mountain Lion, it would be System Preferences > Accessibility.
Uncheck the in the "Seeing" or "Zoom" section that says "Use scroll gesture with modifier keys to zoom". It may be slightly different if you have an older OS X version. I have Mountain Lion and am going off of memory on the older versions.