There's actually a double-edged sword on this point - women consume fewer calories under ordinary circumstances, but when building muscle, have comparable food requirements to men. Due to hormonal differences, women lose muscle faster than men, which adds an additional time requirement for maintaining physical condition, and makes food shortages much worse with regards to decreasing combat efficacy. Not only that, but there are other issues of logistics - keeping troops clothed, shoed, etc - that would unambiguously suffer from the '10% less effective' problem without any significant offset.