This only yields an approximation because the ray origin is placed at the center of the cube, not its surface, which is the true location of the reflection.

Given the cube is centered at the same point, does this approximation method produce objects of the same size in the reflection map regardless of the size of the cube? In this case in real life, the objects in the reflections should be larger if the cube is larger since the reflection surface would be closer to the objects.

@acz, yes. The different colored objects in the cube map would not change with the size of the cube because the forced perspective from the center of projection. I agree that this behavior would not mimic that of real reflective objects.