The effect of steroids, generally speaking (IMHO), is not so much to let many people become impossibly well-muscled, but rather to increase the frequency of well-muscled people in the general population.
A body like Michelangelo’s David is obviously possible without steroids, and represents maybe one or two standard deviations to the good for a twentysomething man. (In case it’s not clear, I’m making these numbers up on the spot).
But human physiology and endocrinology vary tremendously, and very young men with naturally high levels of testosterone have somewhere between four and twelve times as much testosterone as very young men with low levels of testosterone. (These numbers I’m not making up).
And the numbers I just linked to probably represent a realistic range for the general population; in other words, it’s likely that real outliers (maybe 1 in 100,000 or 1 in 1,000,000) have significantly higher testosterone levels than the link suggests.
In addition, I’d bet folding money that there’s similar variation from man to man in their response to a given level of testosterone.
Anecdote I: I used to be a (barely) national-level bicycle road racer. I was a junior (under-18 competitor) in the late '80s and early '90s, and few of my cohort were doping (beyond prescribed amphetamines). There was a huge range in leg musculature; some guys had the legs of high-school cross-country runners while others strode on tree trunks. But there was little correlation between leg muscle volume and either short-term speed (fast-twitch muscle fibers) or long-term speed (slow-twitch muscle fibers/aerobic capacity). There was some correlation, but not much.
Anecdote II: a woman I dated a year ago is a former competitive bodybuilder. Now, she looks like a “normal,” well-muscled woman; her arms are toned but less so than, say, Michelle Obama’s. But when she was competing (about ten years ago) she was extremely muscular for a woman. If you look at a range of women who are “natural” (non-steroid/HGH-using) bodybuilders, she looked about like that. So IMHO and in my experience, most muscular female body builders who claim to be “natural” are probably telling the truth.
Another woman I dated is a competitive distance runner. She has a subtle-but-clear six-pack. And I am as certain as one can be that neither woman ever took testosterone.
Also, keep in mind that body dysmorphia and other factors can be extraordinarily powerful motivators. Some people excercise intensively because they are compelled to, and such a compulsion can have profound effects on one’s appearance.
I’ll likely never have a six-pack, though in my twenties I was close. Many men and women who do have six-packs got there by having good endocrinological levels/response, strong willpower and maybe a hint (or more) of compulsion.
But even giant dudes aren’t necessarily juicing. Some of them certainly are, and as a result there are more of these guys around than there used to be. But it would be foolish, IMHO, to underestimate the range of human variety.