A couple of people have noticed that the font sizes in their browsers, particularly Safari, seem to be changing when they are using them. When browsing, the text and some images may seem to zoom in a little. There are two possible reasons for zooming behaviors in OS X. The first is if the browser's own zoom feature has been enabled, and the second is if the system's global zoom feature has been enabled. If you use a MacBook with a trackpad that supports multi-finger inputs, these features may be relatively easy to activate.
The first and more likely option for text getting larger in Safari is if you inadvertently activate Safari's zoom feature, which will increase font and picture sizes proportionally within the browser window. This can be activated by pressing command-plus/minus to zoom in or out, but also can be easily activated if you use a multi-touch trackpad.
The system will perform a zoom function in the foremost application if you use the pinch gesture on a trackpad, and in Safari this activates the aforementioned zoom feature. There may be several reasons why the trackpad is recognizing a gesture as a zoom, but the most prominent one is if you have your fingers separated when using them to scroll up and down on a web page.
If your fingers are separated, they will more easily spread apart or come together when you perform the scroll-up or scroll-down feature, which can be interpreted by the system as a zoom. The best way to counter this is to make sure the tips of your fingers are together when moving them up and down on the trackpad.
Optionally, you can look into using an alternative trackpad driver such as "BetterTouchTool" (very much in beta development) that will allow you to customize the extents of when the system recognizes an input routine as a gesture. Using this tool you can decrease the sensitivity of the zoom gesture and make it easier to scroll without inadvertently activating it.
Beyond application-specific functions, OS X has a system-wide zoom feature that can be enabled in the Universal Access system preferences, which takes the whole display and increases the size of the display output, pixel-for-pixel. This causes the extents of the display output to be larger than the screen's viewable area, so to allow these areas to be viewable the system will center the display on the mouse position. This results in everything being larger on the display, and the whole screen moving when you move the mouse cursor (either with the cursor or when you move the cursor to the edge of the screen).
While this feature can be enabled in the system preferences, or by using the option-command-8 key sequence followed by option-command-plus/minus to zoom in and out, it can also be activated by scrolling gestures (on trackpads or the magic mouse) as well as the scroll wheel on other mice. Just hold down the control key when scrolling, and the system will activate the zoom feature.
Inadvertent activation of this function may happen more for people who often use the control key and other modifier keys when using the mouse, so be aware if you have the control key pressed. You can turn this shortcut off (or change its activating modifier key) by going to the "Mouse" system preferences and unchecking the option to zoom using the scroll wheel.