Page keeps zooming in for no reason

I open Safari (iMac with Lion) and then start moving my mouse around and for no reason the page zooms in. I can only assume the mouse has a zoom in function that I'm not aware of, and that I've inadvertently stroked it somewhere I shouldn't have, or tapped on it accidentally in the wrong place. Once the screen's enlarged I try everything to make it zoom out again but it wont. I try stroking with one finger, two, three, four, both hands (ok joking about both hands ) then tapping it once, twice, etc, but nothing will return it to its original size....except going back to the previous page then reopening the page in question. Can anyone please tell me how the zoom in and out functions work so I can at least reverse it when it happens unintentionally. Thank you