Subtract subcutaneous fat; and the human male-female difference in muscle size and strength is about the same as for gorillas
Despite claims of reduced levels of sexual dimorphism in the genus Homo (e.g., compared to Australopithecus) (McHenry, 1994, Plavcan, 2001), muscle mass and resulting muscular strength are very sexually dimorphic traits in contemporary humans. On average, men have approximately 61% more total muscle mass than women (Illner et al., 2000, Kim et al., 2004, Phillips, 1995, Shen et al., 2004, Wetter & Economos, 2004).
Relatively more of this muscle mass is allocated to the upper body, with men having about 75% more arm muscle mass than women (Abe et al., 2003, Fuller et al., 1992, Gallagher et al., 1997, Nindl et al., 2002). Not surprisingly, this latter difference translates into approximately 90% greater upper body strength in men (Bohannon, 1997, Murray et al., 1985, Stoll, 2000).
The mean effect size for these sex differences in total and upper body muscle mass and strength is about 3, which indicates less than 10% overlap between the male and female distributions, with 99.9% of females falling below the male mean. An effect size of this magnitude also means that sex—a single dichotomous variable—explains roughly 70% of the variance in muscle mass and upper body strength in humans.
The sex difference in upper-body muscle mass in humans is similar in magnitude to the sex difference in lean body mass in gorillas, the most sexually dimorphic primate (Zihlman & McFarland, 2000).
Sex differences in lower-body muscularity are nearly as large. In the legs, men's muscle mass is about 50% greater than that of women with a mean effect size of approximately 2 (Fuller et al., 1992, Lawler et al., 1998, Shih et al., 2000), and lower body strength is about 65% greater with an effect size of about 3 (Bishop et al., 1987, Falkel et al., 1985, Wilmore, 1978).
These substantial sex differences in muscle mass and strength suggest that there has been strong disruptive selection favoring greater male muscularity in the human lineage. In overall body weight comparisons, a female advantage in fat mass largely counterbalances a male advantage in muscle mass, making the sexes appear quite similar. But this gross similarity masks very different tissue investment strategies by males and females, which, in turn, suggests divergent selective histories, thus undermining conclusions about the human mating system based on overall body-weight dimorphism (e.g., Plavcan, 2001).
From William D. Lassek & Steven J.C. Gaulin. Costs and benefits of fat-free muscle mass in men: relationship to mating success, dietary requirements, and native immunity. Evolution and Human Behavior 2009; 30: 322-328.
NOTE: Perhaps the key statistic here is that 99.9% of females fall below the male mean for upper body strength. What this means is that only about one woman in a thousand is as strong as even the average man.
When talking about much above-averagely strong men - as in elite sports and the military - essentially zero women are that strong.
This is a clarifying fact with respect to many policy discussions; or it would be, if facts carried any weight in the modern world - which they do not.