That makes sense now. Thanks for the screenshots and details!
The 15" at 250% means it has less effective space (for lack of a better term) than the 27" at 100%.
If the Opus window size is saved on the 27" 100% screen, then loaded on the 15" 250% screen, the position and size will be scaled by 2.5x, but that means things toward the bottom/right of the 27" will go off the edges of the 15". (Because the scaling factors are not proportional to the number of pixels on each screen.)
If you turn on Preferences / Launching Opus / Default Lister / Always over the mouse pointer then the window will no longer go off the side of the screen, and instead will open near the mouse pointer while being kept completely on-screen (unless it's larger than the screen allows).
That may be all you need to do to make things comfortable. On the other hand, if you want more fine tuned control, you might find the AreaCommand script add-in I wrote useful. It lets you run different commands when the mouse pointer is on different screens (or in different parts of a screen). So you could make desktop double-click open a different layouts for each screen, or you could make it run a command that opens the default lister but override the size of it.
e.g. Go New=200,100,1500,1000 opens a window at 200,100 which is 1500x1000.