I'd actually have to go through all the css code to find the offending line but somehow a font-size of 11px is being applied to the body element and thus reducing the size of all the "em" sized text in the forum's css file.
At least that's what FireBug says.
Also, the forum is looking narrower because the #wrap div is set to 960px and the forum width is set to take up only 90% of that width.
To make the forum as wide as the banner, remove the
width: 90%;
and change
padding: 1.3em 2em; to padding: 1.3em 0;
in Oxygen.css and to fix the font size issue, change this
font:11px/18px Verdana, Arial, Helvetica, sans-serif;
to
font:1em/18px Verdana, Arial, Helvetica, sans-serif;
in your style.css file and it should be fixed.