Zomming Remote Screen on Android Tablet

I have an 8" Android tablet I am using to remote control an Android phone. The tablet has a screen resolution of 800x1280 and the phone has a screen resolution of 480x854. When I initiate remote control, the remote screen of the phone take up only part of the screen on the tablet, about half.

Now, there is an absence of any tabs or controls as far as I can find on the remote control app to adjust the scaling or zoom of the remote screen so as to make it take up all the available space on the 8" tablet.

So, I changed the resolution of the tablet to 400x640 and then the remote screen takes up about all of the space on the tablet screen. And a "zoom" icon appears on the right side of the screen. The zoom icon allows me to scale the remote screen to further enlarge it on the tablet screen.

In the tablet native screen resolution of 800x1280 the zoom icon is missing from the screen. I would like to have the zoom icon at that time to zoom the remote screen to the full size of the tablet screen without having to manually change the tablet screen resolution.

Is the possible, and if so how would I do it?