I always thought that was related to the training itself more than definition or diuretics. Like heavier, lower rep training helped to develop a more solid/deeper looking physique compared to relatively lighter weight/higher rep work.
Except for some true genetic freaks that will have the look no matter what, I'd say most guys from the Golden Era and guys like Levrone, Johnnie Jackson, and yeah Branch too, had/have a slightly different-looking "finished product" compared to Flex (Wheeler, not Lewis), Rhoden, or Kai.
Guys that built a base of significant strength before getting into bodybuilding and/or who regularly incorporate legit heavyass weights along with "traditional bodybuilding" seem to bring that little something extra once they diet down. Maybe the whole "sarcoplasmic vs myofibrillar hypertrophy" concept has some merit after all?!? But hey, that's just my opinion. I could be wrong.