It's actually because of the greater risk "from breast, ovarian, and uterine cancers" (article). There's a more scientific write-up here, and while I don't have access the abstract implies that a more rigorous update of the exposure criteria upheld sex differences. From a more terrestrial perspective, The Nuclear Information and Resource Service (nirs.org) has this rather incensed piece on the issue. They cite a 2006 NAS report which
finds that harm to women (cancer) is 50% higher than the comparable harm to men from radiation doses that fall within the legal limit to the public over a lifetime.
Basically, it looks like females are more likely to get cancer from radiation, and once they do, those cancers are more likely to cause more harm. According to the executive summary of NASA's 2014 in-depth report:
[A] 45-year-old man has a 344-day limit versus a 187-day limit for a 45-year-old woman, due to radiation exposures on the space station that would be typical at the time of solar maximum.
The effect of body size, however, could be the opposite. Taller women get more cancer, with that study showing a 16% increase in incidence for every 10cm (4in) height increase. That increase, however, is among certain types, such as melanomas, and not the above-mentioned "breast, ovarian, and uterine cancers." Males, being taller, could therefore get more cancer overall; it turns out males usually die more from most cancers, just not (obviously) "breast, ovarian, and uterine cancers."