I'm not sure i understand. But i can add this:
When the GUI is smaller than the set size, then you find an arrow in the lower right corner of the application. When you move the window and click this arrow, then the GUI resize. When the size is the size you have set - like 80% -, then the arrow dissapear.
You can allways set a different size, also by using the computer keyboard, like you can see in the resize menu.
So i'm not sure what you think is wrong.
If things does'nt work like i describe, and a cleaning of Mini Vs prefs and a reinstall of mini V does'nt help, then i suggest you contact Arturia support.
I don't find anything has to be wrong. But it can be DAW or OS depended, in case anything is wrong.