Sorry to barge in, but I can see the other page ( in your storefront ) are nice aligned with the menu, so why is this not happening at the page you refer to: http://sylviabouwer.nl/2016/01/28/wil-je-weten-wat-mijn-nieuwe-plannen-zijn/
Seems to me if storefront aligns it on other pages ( except the homepage which can be set to full widht) why this smaller layout on this one page right?
This webshop is also build with Storefront ( and the Pootlepress Storefront Pro ) and as you can see it is always the same widht on every page?
Did you by any change center the menu on that page? or, if I am not mistaking this is a blog page template? maybe something with the sidebar or setting in the pagebuilder?