Can't enlarge to fullscreen

Hello. I use a medical platform, Semble, for work. However my client view window will not enlarge. When I hit the 'fullscreen' toggle, it creates an even smaller duplicate window smaller than 25% of the entire screen. I know that the viewer window doesn't fill the screen, but it should get much bigger.

I have emailed Semble but had no response. I was wondering if it may be something in my settings? I am on a MacBook Air iOS 11.7.10 and I am not tech savvy - just to warn you!

Any help very much appreciated, as it is really affecting my client work.

MTIA