You can absolutely measure trace amounts of almost any substance in the human body, we have the technology for that. Jones being almost or completely off the stuff is definitely showing. He used to be able to handle very strong men and elite wrestlers with ease like he's some sort of a machine, this is not the case anymore.
And yes, Stipe is a real heavyweight. When he's 240-250 pounds he doesn't even look overly muscular, which means he's just a massive guy without the body fat or much muscle. Cormier is not a natural heavyweight, he's just carrying too much body fat and is not disciplined with diet. If you actually stood next to Stipe, you'd see how big of a guy he really is, by that I mean the size of his head, his frame, his fists. Jones bulking up would just hurt him and if he's having problem controlling lightheavyweights of similar size, it would be way harder with Stipe. I'm not a huge fan of Stipe's fighting style, but he's a monster and Jones has very little chance of winning.
As you may know, people in the Netherlands are the tallest people on Earth overall because whole countries are taken into account. However, if we're talking simply by region and certain genetic haplogroups which are dominant in certain areas, the tallest men on earth live in southern Croatia, Bosnia and Herzegovina and certain parts of Serbia and Monte Negro. It's not uncommon for 6'1 guys feeling like manlets when they go there for a vacation. And it's not just the height, people there have massive frames and similar built like Stipe (similar jawline and forehead too, with huge heads like him). Dinaric Alps are the home of many giant people.
Jones is probably the best fighter of all time and his octagon IQ is second to none. However, there is a lot of direct and indirect evidence that he has been doping hard for most of his career. Do I think he's a better fighter than Stipe? Yes. Do I think he has a chance of winning against Stipe? Only if Stipe ages over night in fighting terms, otherwise I think Stipe wins 8-9/10 times. He's just too damn strong and tough and probably more skilled than most HWs. Not only do his fights show that, but many sparring partners (even before he was even a contender) witnessed how strong he is and how his short and perhaps seemingly not so powerful punches are devastating. Weight classes exist for a reason and Jones doesn't have much chance at HW. Him putting on more muscle would only hurt him and would certainly not help him much against monsters like Ngannou, Blaydes or Stipe.
But hey, if Jones moves to HW and is not successful, you can always blame it on lack of motivation and wear and tear.
