It is possible that your phone screen is mirrored on the tablet but you do not see it on full screen. You can see the image displayed smaller than expected.
To display the image in full screen, you need to adapt the screen display on your phone's settings. Choose the highest resolution to begin and check if the image is displayed in full screen on the tablet. Then you can choose a lower resolution if you want to and if your tablet displays the image in full screen.
Below is an overview of the menu in which you will find the screen display for Android phones.