A molecule of steroid binds to a receptor. Two receptors bind together to form a homodimer which then binds a "receptor" on DNA called a hormone response element. These response elements can either increase or decrease the expression of the genes that they regulate. So, for every nucleus (muscle cells are multinucleated) you have the genome of DNA and the "receptors" (HRE's) for the steroid-receptor complex. As you increase muscle size, you also increase the number of myonuclei and therefore, the body increases the number of androgen receptors to ensure that there is adequate stimulation per unit of DNA (so for example, you increase one unit of DNA and one unit of receptor but there is still a 1:1 ratio). This is the "upregulation" that a lot of papers mention. There is also another form of upregulation where the number of receptors per unit DNA is increased - this is true upregulation and occurs in the absence of androgen (for instance, castrated animals, women, and chemically castrated men). It is the bodies response to try to increase the likelihood of androgen receptor activation and The corollary also occurs, that is, the number of androgen receptors per unit DNA is decreased -- this occurs in response to androgenic stimulation. So, in the end, you could get an increase in the number of receptors but still have a net downregulation due to the ratio of androgen receptors to unit DNA actually being lower.