You have a table cell with a width set to 600px:
<td width="600" rowspan="2" valign="top">
<div class="main-container col2-left-layout">
<!-- More HTML -->
</div>
</td>
Find the template that contains that markup and remove width="600"
and it should work as expected.
Bonus advice you didn't ask for: I highly recommend not using tables. Tables are for tabular data, not layout. Anything you can do with tables for layout can just as easily be done with CSS. Maintaing a site built with tables is not only non-semantic, it becomes extremely hard to maintain over time, especially when you're building a site with Magento which has so many moving parts.