I am with Isaac on this one. Competitions like the IMO are all about numbers (duh), not only in solving problems but in team success. For a country to do well, you need a very large population exposed to a rigorous curriculum, from which a tiny number of potential champions will emerge. Worldwide I just don't think the number of girls participating matches the numbers for boys---not yet, anyway. Add to this the fact that social pressures, even today, still push girls away from math, and you should appreciate our caution regarding gender generalizations.