As the castrato's body grew, his lack of testosterone meant that his epiphyses (bone-joints) did not harden in the normal manner. Thus the limbs of the castrati often grew unusually long, as did their ribs. This, combined with intensive training, gave them unrivalled lung-power and breath capacity.

Operating through small, child-sized vocal cords, their voices were also extraordinarily flexible, and quite different from the equivalent adult female voice. Their vocal range was higher than that of the uncastrated adult male.

Listening to the only surviving recordings of a castrato, one can hear that the lower part of the voice sounds like a "super-high" tenor, with a more falsetto-like upper register above that.

:jesus-christ: the practice warped their bodies too.

Example of the bone changes:

https://hexbear.net/pictrs/image/FCdBRHwVyY.jpg

Paper on the biology / forensic physiology of castrati:

https://www.nature.com/articles/srep28463

They apparently almost always grew over 6 feet tall