The polygon sizes must be proportional to each other, so when one is small, it should be small on the UV too. For example, on the inner shoulder, the outer polys are quite big compared to their size on the actual mesh. And so, polygons will try to fit a large portion of the texture within a small area, which causes texture squeezing. And vice versa, small UV polygons will stretch the texture if a 3D polygon is big.
I can imagine a function or an addon that would make the mesh-UV proportions equal, although I didn’t look for such.