To balance out the rest of Synthol Guy’s body to match his arms, he’d have to look like the after portion of the animated gif below. Not only would his forearms have to keep up with his upper arms, his whole body would have to be developed as well, including his neck. However, that would leave him with an endo-ecto head. A body with the natural genetics to get that big would also have the facial bone structure to match. To top it off, his upper arms would have to have the same coloration as the rest of him instead of the flush, circulation-impaired pinkish color they currently possess.
On a side note, Dave Palumbo has discussed a study which provides evidence that the original synthol actual builds muscle: