Can anybody really explain what a 'head voice', a 'chest voice' and a 'falsetto' are. I suppose if I ever had attended some singing lessons I would know the difference - depending on what my teacher knew, and I'm a little biased here.
I can also use my 'head voice' (or am I singing 'falsetto' when I do?) but not loudly and I also can't do smooth transitions to/from my normal ('chest'?) voice. But I think that's simply a matter of enough hard practising (plus talent), and so I have some great respect for everybody who really can sing that way.
I DO NOT understand how the difference between boy and girl sopranos should be the difference between head voice and chest voice. On the other hand it is VERY easy for me to understand how mixing two voices singing at the same pitch but with different vibrato frequencies can yield VERY strange results.
And - there's the aspect of castrati. As leeneia said the sheer size of what is producing the sound in question matters - men simply become larger, also when castrated. Second: If a boy is to undergo that procedure he must be a real talent anyway. Third - once he's done with it: What should prevent him from concentrating on what he's living for? (Irony? Yes, very bitter irony.)
So the third aspect might simply be the amount of practise spent at some age when it really has an effect. Just imagine the difference between a girl of 18 trying to become a soprano star and a castrato a the same age... (no joking intended). I wouldn't be astonished if it turned out that castrato sopranos really WERE better than female ones, especially as soloists (solists?)
But - hehe - to imagine that ability (aquired by supported (argh! in this case) talent plus hard work where others are ... whatever ...) is more likely to make you a star than sexual ... whatever ... is also some charming idea to me.
May you all never be 'supported' that way! (What's that in irish?)