It could have to do with many different factors, including:
- The padding/margins of the body tag
- The aspect ratio of the images you are uploading
- The final calculated resolution of the site on the mobile device
I would recommend trying another theme or modifying your current theme. Another solution would be to hack the catablog-lightbox.js file in CataBlog. The problem with the later solution is that any updates will overwrite your changes.
Good luck and let me know if you interested in more specific instructions on hacking catablog-lightbox.js.