Issues with Portfolio "wrapper"

Hi,

I have a problem with my Portfolio page. I am using a child theme, and have changed the general container size that holds all the sites content to 720px. When you scroll through all the pages there are no problems and they all have the same alignment, except when you come to the portfolio page, here the wrapper expands for some reason. Does anyone know how to control this or change any of the code so this doesn’t happen? Here is a link to the problem page: http://kidberlin.com/books/

Thanks in advance, Mike