colorbox content wrong size
In ur html rel="gallery-all" is available which means you allow to load all the images in the galery or page, for resolving your issue (for single load) remove this rel attribute.

The rel attribute specifies the relationship between the current document and the linked document.In colorbox "gallery-all" means it will load all images to colorbox. For eg here load 4 images, perhaps select "single" load the main image