So what does someone on testosterone look like? Sorry to burst your bubble man, but the dude was using any form of substance as I stated, cocaine being a great substance used to cut, even most likely used by the OG bodybuilder Eugene Sandow. There is no "genetics" factor to bodybuilding you either roid and get big or stay natty and be small, it's really that simple. Steve Reeves was on things, what, I don't know but in an era where bodybuilding was barely a thing and they didn't even have a proper working out method or any idea of eating right he had to be on something, whether drugs or steroids or both.
Um...I don't have a bubble to burst. I've been around too long and seen enough 'roided out pigs to know what they look like. On top of all that, there is no evidence except for hearsay and the random "testimony" of those that did take them and wish to divert the truth of their physiques away and on to Reeves. Example? I do recall it being reported by Sergio Oliva that he took steroids with Reeves. LOL!
As stated elsewhere, Reeves was well built at 16 and continued to improve over the years. Not instantly, but over the years. Reeves was also tall and a person that takes steroids prior to reaching puberty tends to be shorter than average because the onslaught of testosterone in a human male signals the body that maturity has been reach and the epiphyseal plates seal. Reeves was 6' 1". Just look at the average dope builder of today and most are much, much shorter. And dumber.
You can choose to believe what you want but that does not make it so.