Hi @nicovon, I just tried a test out out and this is working for me when I share a screen or application.
We're currently running Webex version 40.12.5.14
I used two monitors
After starting sharing I am able to expand the size of the small video window right up to full screen of my second monitor. The initial layout of this video window appears to be linked to what layout was being used prior to the share beginning. You do have the ability to change the layout (top right of the video window) once sharing so it's not something that has to be done prior to the share being activated. In my case I was originally on grid view although I have tested the other layouts now as well. With the video window in it's smallest size it will show a 2x2 grid and as you expand the size of the window the grid size increases.
If you need screenshots or any more info let me know.