The proportions are what makes it so cool looking. They're not insanely skinny as the final puppet yet they deffinitely aren't human. It deffinitely looks more alive and solid.
look at the tighs and feet, they appear way more powerful and believable for a speedy creature. The torso is uniformly slim, with the waist continuing seamlessly in the flat and form fitting rib cage up to the shoulders like the Winston aliens did, and it has a smaller back, almost in proportion with the hips, giving it the femenine look, while keeping the thick shoulders like the Giger Alien.