What app are you using to open the prototype on the phone? If it’s a mobile web browser or an app using a mobile web browser rendering engine, it wouldn’t surprise me if the rendering/resizing of a fullscreen image was affected?
What kind of testing are you doing? Would a prototyping app be more useful? There are a bajillion dedicated mobile prototyping apps out there, but I can put in a positive vote for Sketch (if you’re into it - https://www.sketchapp.com/), InVision (http://www.invisionapp.com/), Marvel (https://marvelapp.com/) and POP (although it doesn’t seem to play nicely with the older phones anymore - https://popapp.in/). I’ve also heard good things about UXPin for mobile but haven’t actually used it myself.