You are absolutely right! I was confused between the QGraphicsView and the QGraphicsScene. I just realised that when I was playing with my POC.
Actually, what change the size of the QGraphicsScene is done in another part of my code and is called later. If I recalculate the sceneRect according to my needs and call setSceneRect(), when my window is resized, everything works as expected.
Sorry for the inconvenience.