On average, men have 61% more muscle mass than women (d=3), a sex difference which is developmentally related to their much higher levels of testosterone. Potential benefits of greater male muscle mass include increased mating opportunities, while potential costs include increased dietary requirements and decreased immune function. Using data on males aged 18–59 years from the third National Health and Nutrition Examination Survey and including other relevant variables, fat-free mass (FFM) and/or limb muscle volume (LMV) are significant predictors of the numbers of total and past-year self-reported sex partners, as well as age at first intercourse. On the cost side, FFM and LMV are strong positive predictors of daily energy intake and strong negative predictors of C-reactive protein and white blood cell count, measures of native immunity.
Could be that testosterone is the mediating factor here, causing both greater lean mass and a higher sex drive. Of course, a higher sex drive does not equal fulfillment of that drive, so something else is at play, perhaps women finding men with higher muscle mass more attractive. From a game standpoint, there’s also the fact that T induces greater self-confidence, lower anxiety, and higher aggressiveness, all qualities that will help add to the number of men’s sexual partners, all other things equal.
The paper’s authors note that greater muscle mass has potential costs, such as greater dietary requirements and decreased immunity. However, in this modern age, food is cheap and plentiful, as we see by the obesity epidemic, so it’s not much of a cost. As for immunity, yes, that is a cost, but lower inflammation comes with it, so that might very well decrease the incidence of inflammation-related illnesses, such as cancer and heart disease and, in fact, virtually all of the so-called diseases of civilization.
On the whole, it looks like higher T has a low risk/benefit ratio.
Lower WBC count and C-reactive protein, in the above paper, represent the costs to extra muscle. These are also indicators of inflammation and, ceteris paribus, you want them lower.