It was noted that the Terminators (T101) were infiltrator units, designed to infiltrate the resistance.
Given the ragged undernourished humans of the time, wouldn't making an infiltrator body into the hulking over-built Conan The Destroyer frame be something totally illogical if your goal is to blend in?
Why not use a more "normal" body? Robots already have metal frame and super-strong muscles, so size wasn't required to bump the combat capacity as with flesh and bone humans.