Well, it's actually not so easy. Since it's part of the grid, reducing the margins there would do it everywhere and also mess up widths of the colums (since it's all based on percentages). The easiest way would probably be to use a Full Width page type, then add a Layout to the main area and specify what size you want the margins within the layout.
http://www.concrete5.org/documentation/using-concrete5/in-page-edit...

Haha! I knew as soon as I wrote that it'd be easy, I figured you say it wouldn't be!

I understand how to use layouts.

It's too bad you can't add one to a page_type default.

Thanks for answering tho!

No problem!