As every medical student comes to learn when first dissecting the human body at medical school, our limbs are almost entirely composed of muscles.
It is likely that no further improvement in muscle power can be achieved by increasing the density of packing of the myosin motors.
How is it that an ant appears proportionately so much stronger than a trained human weight lifter?
Only an organism of our dimensions and android design — 1.5 to 2 meters in height with arms about 1 meter-long ending in manipulative tools — can handle fire.
Being terrestrial is one obvious requirement. No fully aquatic species could master fire and thus develop metallurgy.