Even so, having high levels of T generally makes it easier to put on muscle mass than for individuals with lower T. Still, even that's uncertain because men differ on how their bodies process the stuff. Some men may have more testosterone receptors, which would probably improve responsiveness. Others might have a higher clearance rate of Testosterone, which would probably decrease responsiveness. And, another group might have very high levels of bound T, but very low levels of free T (the stuff that's biologically available for growing muscle and the rest of the stuff associated with T).
This can be thought of in the following way: the hypothalamus in the human brain has a detection mechanism which regulates the release of many hormones which control the production of androgens by the testes and the adrenal glands. If this detection mechanism detects an abnormally high amount of androgen / anabolic steroids, which it recognises primarily as testosterone, then the hormones responsible for testosterone release will have their production decreased, and so the testes etc. will no longer be stimulated by these hormones, and so will cease, or decrease, production of natural testosterone, and so cause testicular shrinkage.