SCIENCE ALERT! BIG-OLD BLOCK OF TEXT! YOU HAVE BEEN WARNED!
I thought this one was pretty much understood. Humans are sexually dimorphic, and greater strength is more prevalent in males.
Humans are mildly sexually dimorphic. In animals, sexual dimorphism is related to the amount of required parental involvement in child rearing. Animals that are not parentally involved, or that distribute child-rearing among social groups are less-to-non dimorphic, compared to animals with extensive parental involvement, or with young that are exclusively raised by one parent. Human children require an incredibly high-degree of parental involvement (one of the highest of all animal species, thanks to the
Obstetrical Dilemma), but this has been a relatively recent change in our genetic history. We haven't had the evolutionary time to make our species heavily dimorphic yet, and with the decreasing prevalence of natural selection on human evolution, we may never be.
Humans are not as dimorphic as similar animals are, but male humans tend to have slightly larger bodies than females. Males also store slightly more of their food-energy as muscle tissue instead of fat, producing muscle tissue even when not exercising, whereas Women produce muscle primarily when physical activity encourages it. In a natural setting where Men and Women hunted to survive, or in cultures where both Men and Women do physically demanding work, this difference is not a very significant one. However, when there are strong societal pressures for Men to be strong and do physically-demanding work, and for women to do the exact opposite, this difference is greatly exacerbated,
and gives us test results like those seen here.
Let's take a step back, and consider it this way:
Given equally rigorous physical training, Men and Women would have the same proportional muscle mass, leaving only the size difference between male and female soldiers. In an age where a soldier's health, awareness, and equipment are far more important than their raw strength, the larger size of the average Man can be seen as a detriment to the Military. Men make for larger targets, are harder to accommodate in vehicles/barracks, require more food and supplies per soldier, and are significantly heavier to carry/transport when wounded, as compared to Women. A given military force composed of Women could be larger than the same force composed of Men, due to requiring less resources and logistical support. Also, as each soldier represents a military asset, the relatively smaller female soldier would be easier to carry from the battlefield when incapacitated, and thus would be easier to preserve, making an all-female military a wiser investment.