Image Size does not change

Hello,
I’m using your plugin on a production site as well on a testing page. They are 1:1 copies (wordpress, plugins, settings etc.) of each other, except of the webserver / OS which is different. On my production site the lightbox setting for changing the Image Size does not make any difference, it does however work on the testing page. It does also load the wrong image on the production site, where it works on the testing page. So I’m not sure where to start looking at and if this can be in some way related to the base OS / webserver installation.