It's the margin bottom on element
<div id="page" class="hfeed site">
from this style:
body .site {
margin-bottom: 3.42857rem;
}
Removing this style gets rid of the space (and as far as I can see doesn't cause any other display issue with the site.
You can find these kind of issues by using the Developer Tools in your browser (the Firebug plugin for Firefox for example) and just right-clicking on the area with the issue and exploring and dynamically editing or disabling the styles that are applied to an element.