Display size problems with two monitors
That's what it looks like and that's the behavior when you zoom in on the laptop screen.  You can zoom in (even under Snow Leopard) via the trackpad or scroll wheel or keyboard.  In Snow Leopard there settings for screen zoom in System Preferences under "Universal Access" (both "Zoom" and "Zoom Options..."), "Mouse", and "Trackpad".  See what you have configured and use that to unzoom the screen.

I think the default zoom is ⌘+scroll wheel.

You have been able to zoom in on the screen as part of Universal Access for as long as I can remember.  You still can in Lion.