The ones that appear to be smaller actually have all the design elements of the larger ones, and are drawn to the same scale. Essentially the same technique can be seen in the khorjin face shown here. But the only significant difference among them is color. As in the balisht, design elements are rendered nearly invisible in some of them by working them in black against a deep indigo background. It seems reasonable to assume that the weavers were well aware that this optical effect would occur.
Several possibilities come to mind.
Although weaving a repetitive pattern, a much less repetitive, livelier, more interesting result is achieved by simple manipulation of the colors. Secrecy is an important element in much tribal custom, and the nature of the secrets is of much less significance than the fact that only certain people know them.