Second Level Forum UI

(To make myself more clear, I'll use Breadcrumb to show the level I am talking about.)

I set up a forum that has several sub forums in it. I styled the forum page (Home > Forums), but the sub forum page's forum (Home > Forums > Forum) looks narrower. When I view the source code, the forum UI is using a table. I also use Firebug to see if any styles can be influencing the width, and there isn't any. The containers outside are all the same in "Home >Forums" and in "Home > Forums > Forum", so nothing is pushing extra padding to force it being small.

Does anyone know what's going on? Is it possible to solve this problem without further CSS? This Forum will grow bigger and bigger, so I don't want to style the table width with specific ID name such as "forum-topic-1", "forum-topic-2", etc.

Thanks in advance.