It doesn’t have to be two monitors. Two windows on the same screen exhibits the problem.
Create new project and add a second window
Note both are zoom 200%
Change zoom on first window to 300%
Second window no longer full
Perhaps we should set up a GoFundMe to buy you a second monitor… 
M4 Mac Studio
Sequoia 15.4.1
Dorico 6.0.0.6026