Portfolio category changing page layout

Hi,

I really love the theme but have a question about a site I’m working on using a child theme.

I just added a Portfolio page ‘Books’ to the main menu (4 column page), but the new page doesn’t keep the same dimensions as the overall layout, and the width of the new Portfolio page has expanded. The example is the correct layout http://kidberlin.com/news/ and the portfolio page http://kidberlin.com/books/

Does anyone know what could be causing this, or how to fix this issue so that the portfolio page keeps the same alignment as the other pages on the site?

Many thanks in advance,
Mike