Go on, resize your browser font size now! (Note: Yes, when you resize the font enough, the navigation will wrap into two lines and look a bit rubbish, but this is what you get for using horizontal navigation)
The navigation "shadow" is 3000 pixels wide to support almost any resolution (this one is resized):
You could use a 1 pixel gradient and repeat it in horizontal direction, but that does not allow for positioning the shadow.
The tab images are much bigger than necessary: