This is intentional, yes. The logic is that since areas outside of the camera region are no longer visible, the compositing space is now limited to the camera region, which has the same aspect ratio as the render size.
@EWA Technically, there is no such heuristic for the regular compositor, since the choice of its output nodes is context sensitive, not prioritized, depending on whether it is executing as part of the render pipeline or the compositor editor.
However, your concern makes sense, so I went ahead and changed the priority in: