Currently, OpacityMask resizes and moves the objects involved in the Mask.  Based on how Masks work in all standard Design applications, it shouldn't do this.  

Currently to get OpacityMask to work the same way it does in Figma, Illustrator, Photoshop, XD, Sketch etc, I have to add mask and masksource to their own Items and assigned a height and width to the item that matches the parent.  If I don't do this, OpacityMask will resize the output.  Attached are some visuals to helps explain.

If you look closely at what is happening in QtCreator, the OpacityMask resizes the 100x100 red rectangle to 640 x 480.