Why do so many of the “clay ladies” of the ancient world have triangular arms?
Let me mention three—not necessarily mutually exclusive—possibilities.
The first is schematization. For the sculptor, especially in friable clay, arms are problematic. Detached arms invite breakage, but when attached—held against the sides of the body, say—they tend to disappear visually.
So stylizing the arms into triangles overcomes both of these problems, while still faithfully depicting the body in its fullness. It's worth noting that, in many of these figurines, the legs and feet have also been schematized into a single, triangular form.
(Remember this salient point; we'll return to it later.)
A second possibility is that what we're actually seeing here are bent arms. A cursory glance over the corpus of these figurines will show that, in a certain number of them, the woman has her hands cupped beneath her breasts, offering.
In this case, the tip of the arm-triangle would actually depict, not stylized hands, but elbows.
Lastly, let us take a step back, and view the triangular arms as part of a visual entirety. Thus viewed, the whole body below the head is outlined, triangularly, by its three outermost points: the two arms and the joined feet.
Thus the body, viewed as a whole, reflects—in large—the sacred delta at its center.