I set the upload resize limit to 800x600. A couple of users were able to upload larger dimensions, i.e. - 2048x1536. No matter what I upload (2550x1905), my large images are set to 800x600.
At the demo, this stuff works fine (trims everything to 1920x1080) and I don't have any clue on why is not working on your website and surely I won't know until you fill the proper bug report and put me in context because I can't help you at all if you don't even indicate which is your website URL.