The reason will be that the screen-resolution from the developement-machine is different to your machine and therefore you will get the empty areas. You could use the zoom-settings to make your output bigger whereby you will get quite unhandy scroll-bars with it.
It couldn't be fixed in a way that all users with probably quite a lot of different monitors wouldn't have such empty spaces and/or too big/small objects and/or scroll-bars and everyone is happy - it's always a compromise.
Even with Qlik Sense which supports responsive design and which could therefore adapt the objects to the screen you will need a modicum of compromise because just filling the screen doesn't mean that's always sensible from a logical and usability point of view.
- Marcus
