You have the CSS #pageWrapper{margin-bottom:30px;}
which displays normally at big screen sizes. However, that space is still there when you resize the window.
The solution is to add margin-bottom:0px;
into the @media
thing:
@media only screen and (max-width: 800px)
#header, #pageWrapper, #footer{
padding: 30px 20px;
margin-bottom:0px;
}
EDIT:
margin-bottom: 0px !important
in the @media
query should do it.