mouse gesture causes sudden increase in window size

I'm unsure where this question belongs, I have searched high and low but obviously have not struck the right combination of search terms. So if this is not the correct place please point me in the right direction.

I generally have the Safari app opened to almost fill the whole Mac screen and have 5 or 6 tabs opened ready to look at each on in some detail.

Sometime when searching the web with a few windows open, using Safari 5.1.4, Inadvertently my finger(s) brush the top of the Magic Mouse and the primary open window will zoom in [enlarge]. At least I think it is a light finger touch which causes this, unless it is a slight movement of the mouse itself. So what was visible, ordinarily normal, on a single web page is now extraordinarily expanded and only a small portion can fit and will appear on the web page. This happens when I go to attempt to drag the window up/down/sideways but I never get to that action. I have no keys pressed on the keyboard [Apple Bluetooth]. This anomaly does not increase the web page window size to fit the screen.

I have tried the CMD - (plus) keystrokes to no avail. Similarly the CNTL - scroll wheel will not bring the page back to correct [previous] size. The only solution is to CMD-W that page and reopen to normal size, in effect, start again.

I have no third party software on my Magic Mouse and I can not see any gesture to do this on the System Prefs for Mouse. I can not re-create this happening no matter what gesture I use on the Mouse and thus am unable to identfy the cause. The zoom happens so fast I can not tell where my fingers were at the time. I can find nothing relevant in Sys Prefs>Universal Access>Mouse>Mouse Options. I have Sys Prefs>Universal Access>Seeing>OFF checked. AFAIK I have no third party apps on my iMac which have any bearing on zoom. I have googled but as usual google results depend on the search terms which I seem not to be able to accurately describe.

24in iMac 8,1 6GB RAM, OS X 10.7.3.

I would appreciate any advice.