The real problem was the .container CSS for the navbar-nav. It contains margins top and bottom.
However, I don't know why it is happening on your website, unless you are using an older version of this theme?
I think the problem has been fixed in the theme, as I don't see that problem on my website, which is using the latest version.
Anyway, I've fixed your site by adding the CSS to correct the problem and removed the CSS I added the other day, which is not needed now.
So all is well.
If you have anymore problems with the KLASS theme please use this discussion thread.