This tool is misleading. It sizes the site according to your CURRENT browser size then lays the semi-transparent graphic over it. This means that if I have a 900px wide content area centered in the viewport, and my current width browser width is 1400px, it appears that there is 250px of empty background on the left and a 380px wide browser would see almost nothing. That's inaccurate.

tl;dr: take this tool with a grain of salt, resizemybrowser is a much better method.