On my website here if you set browser screen size to 640px width and click through on the first button, Win a Toolbox, this then adds an extra 20px to 30px on the width?!
The validator has a few issues with your markup, which I thought might be messing up. When I make my browser window narrow enough all the text on that page seems to increase in size, to the size of large text already on the page. This could be caused by elements leaking in to each-other, and could very likely have other symptoms such as increasing padding or margin in places.