The forearm on the mechanical arm is too short and quite possibly not massive enough, IMHO. The models on pg 1 and the drawing on the top of this page suggest to me that the whole arm slightly over-sided in both mass and length. Plus PP seems to get the mass of the mechanical hands on the heavy side, if that makes any cents, and the anatomically correct proportions that you've got here look 'dainty'.
Again, as I've hopefully said before, please take as a constructive criticism from someone who is in AWE of all of your work and has little of your talents.