Problem with modal dialog and large content.

I am trying to use jquery dialog on my webpage: (click on Apply now
to see dialog)
http://www.ratebot.ca/personal/blog.php
but it does not shade the background content properly when content is
too large. Like in the case above.
It works fine when content of page is small:
http://www.ratebot.ca/personal/testimonials.php
Is there any fix or workaround available for this.