This is known phenomenon, quite normal. The film rails hold the film slightly away from the film gate. So there is a slight gap between the plane of the gate and the plane of the film. The more the rear element of the lens protrudes into the body -- the closer the rear element is to the film -- the more of an angle that the light strikes the outer edges of the film. One result of this is "cosine square law" light fall off, a darkening of the corners that is particularly problematical with digital sensors, which like the light coming straight in. Anyway, as you can visualize the light hitting the outer edge of the film not at 90-deg but at a shallower angle, it is less shaded by the film gate, and "sneaks" by, making an image on the film slightly larger than the film gate. This then narrows the spacing between frames, and extends the image a little closer to the sprocket holes too.