The real killer is mass and bone density. When societies get large enough to make serious war upon each other, you end up with thousands-large shoving matches with blades and shields, and in those scenarios an extra 20 lbs and fewer broken bones per soldier adds up.
I mean, 'is' in the sense that there's still a difference in mass and bone density between the sexes; if we went back to mass shoving matches now that issue would remain for mass participation of women in warfare. 'Was' in the sense that it's less relevant in warfare today as it's actually waged, ie with guns.