The top physics kids in the US tend to get picked up by financial (or some other industry) firms because they're really really sharp because making 200k sounds much better than making 30k. This means that the kids who go into physics are doing it because they truly love the subject but it also means that we have a much smaller percentage of kids going into graduate studies than otherwise. This, combined with the fact that many countries are rapidly industrializing and newly able to support an academic class means that lots of international students are applying to Harvard/MIT/Stanford/etc where some of the best science in the world is happening. Some of the foreign kids go back, some stay here. It really depends. As for makeup? probably 40%-50% of my entering cohort was foreign and probably 60% of the foreigners were asian. That's not really a bad thing though. They're brilliant scientists and it's truly a pleasure to collaborate with them and everyone else in my programme as well.

Disclaimer, I'm a physics PhD student at Harvard and go to seminars a lot with kids from MIT.