Samsung monitor shows black bars on sides

I'm running OS 10.8.2 on a mid-2012 MacBook Pro, and I recently bought a Samsung Syncmaster SA300 (22 inch) monitor which I've connected using a VGA cable (DVI wasn't working, don't know why). I immediately noticed black bars on the sides of the screen, and the picture seems squished, despite the fact that I've chosen the proper resolution for the monitor. Funny thing is, when I boot into Windows 7, the bars disappear and the resolution looks perfect.