I don't think it has much to do with image scale. These old images have a lot of diagonal "hatching" in the backgound which was a way of doing shading with no color or grayscale. I think its like when a subject wears a certain patterned outfit--the fine diagonal lines interact with the A/B scan and you get an aliasing effect that causes the image to flicker.
But I'm not really an expert on such things.