Scale mode adjusts the layout zoom whereas crop mode doesn't. Scale mode only shows more of the layout if the aspect ratio of the window changes, whereas crop mode always shows more if the window is bigger.
The feature is designed for mobiles and tablets with different sized screens with roughly the same aspect ratios (no extremities anyway, like you can get by resizing a browser window to weird shapes). You could try adding your own black bars with some black Tiled Background objects on the top layer.