To be fair, he obviously has resorted to using synthol in the last few years to bring his arms up.
But the rest of him used to be freaky.
2004 BFTO
My opinion is he had a mainline to top-quality pharmaceuticals, trained religiously for the first 15 years of his career, and was extremely strict with his precontest diets (but lied about his off-season diets in his book). The rest is his wide collarbone and quick-bulk genes.