When I do this with for instance a Finder window, it just "zooms" it. You can get the same effect if you go to the Window menu -> Zoom. "zoom"ing tells macos to make the window fit the content that's inside of it, however the app feels like doing that, even if you damn well just want the window to be as big as it can be.
BUT, option + double-clicking any window corner will actually make even a Finder window take up the whole visible space of the screen without being "maximized" (without creating a new screen / workspace).
(double-clicking any corner will make the window expand all the way towards the corner you've clicked; if you have a finder window in the middle of the screen and you double-click the NE corner, it'll get bigger in the N and E directions until it hits the menu bar + the right edge of the screen.
similarly, double-clicking any edge will make the window expand all the way to the border of the screen in the direction of the edge you clicked, and option + double-clicking an edge makes it grow both in that direction and the opposite.)
completely undiscoverable, I feel like I'm lost while Maniac Mansion, just trying every possible Verb + Object (+ Indirect Object) combination to try to read the game dev's mind.