It might not be a schooling thing. China also has far, far more people. The only comparable country is India, and China has consistently out-competed India. I'd be curious why.

The part where East Asian countries seem to consistently dominate the top ranks in recent years with top entries from substantially smaller countries like Singapore and Japan seems to suggest that it's not just a population thing. I wouldn't jump immediately to culture though, because that's a lot of distinct cultures to lump into one bucket.

Wouldn't it be more about a massively larger pool of talent from which to draw?

Long school days, intensive training and drilling of exercises, exposure to 'advanced' math at a younger age, loads of homework, no free time, better disciplined students, and very little life outside of studying - especially in the later few years of high-school for any student interested in gaining acceptance to a decent university.