How come my views suddenly take full width? I don't know what I've changed but now 2 of my views are full width.
Here's 2 screenshots from a view how it was and how it is now.
How can I change this back to normal?
Though it is cut off in the screenshot you have above, the spitfire.jpg image is forcing that cell to 968px wide (968x478) which is larger than your main cell. 

This is more a CSS question than a Drupal one so to bring it back to Drupal, I would recommend that you create an image style for this imagefield so that all images are resized according to the width of your main cell. Check out the image module documentation for guidance on how to create styles, you would then just need to tell views to use your new style for your view. Alternatively, you can use one of the existing styles (ex. thumbnail, small, medium, large) to have views display an image that would fit on your page: