It looks like you may have located the problem, though the fix would be a bit different.
The stock tpl_main_page.php has no tags at all on the center column <td>:
HTML Code:
<td valign="top">
<!-- bof breadcrumb -->
<?php if (DEFINE_BREADCRUMB_STATUS == '1' || (DEFINE_...
100% there is wrong, but there should be no need to specify the width at all.
Apparently there is an added table (not stock) just inside the center <td>, which is properly set at 100% width (though it would be better coding to follow modern practices and use a div with an id styled in the stylesheet).