Not necessarily. A camera that fits the reflecting surface viewport will only render as many pixels and whatever part of the scene as specified, exactly the same as the main camera. Think racing car rear-view mirrors for historical examples. You don't render the whole scene and then cut out a small part for the reflection.
They also don't need to be planar, you can warp the "camera" in any way you want. A monotonic warp function is the easiest, but you can can also map very strange irregular warps. It has been used since the ancient past for Dual Parabloid Shadow Maps for example (which are e.g. in Oblivion).