To add to the sibling comment, it is real. But it's more about the field of view of the lens and the depth of field. As pointed out the light doesn't know the area of the sensor. Depth of field is a matter of the distance from the camera to the subject, the aperture of the lens and the focal length of the lens. Larger formats allow you to stand closer to the subject while retaining a wider view than for a smaller format camera. Take the 80mm f2.8 lens on a Rolleiflex TLR. If you stand where you can frame a head and shoulders portrait, you're standing closer than you would for the same framing on a smaller format camera with an 80mm lens. You would have to stand back further, with the subject further away more is in focus. Have a look at the Brenizer effect for a digitally stitched equivalent of a larger sensor.