At first glance, it looks like you are using Easy FancyBox in combination with another plugin: UnGallery. It looks like the thumbnails are currently linked to a larger version passed through that plugins phpThumb.php file with rather limited size settings. Landscape images all seem limited to 440px wide.
As they are opened in FancyBox, that exact size is re-used as FancyBox which is expected behaviour. The only thing you can do to solve this, is make the thumbnails be linked to larger images.
Is there a setting in the UnGallery plugin that allows you to increase max width and height of the linked large size images?