Would it throw off the fonts and spacing if the table width matched the window width?
Oh yes it would.
The best example is the current instruction page. The wider the window is, the harder it is to read because your poor eyeballs have to travel all that way across the screen to read the text. People with low resolutions will be O.K., but people can afford better monitors now, and it looks worse the bigger the window is. And if your screen is a wide screen monitor to start with, having a low resolution doesn't even help. So, having a set width ensures that almost everyone gets more or less the same browsing experience, and it always looks the same.