Go to 'Design | Library | Layouts' and have a look at 'Web Page Inner Layout > Standard Section Page'. Is there some table code surrounding the 'ParentSectionListTop' that could be restricting it's width.
Also check out 'Parent Section Lists > Standard Parent Section Lists'. This is the code of the list itself. Click the orange 'click here to edit list layout settings' link and see if there is any code there.
Failing that, I would need to see the actual page to make any further suggestions.
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