I've noticed this with my cats over the years and with the cats of friends and family. Quite often, though not always, the cat will tuck their head down and under, perhaps into a pillow, a gap in a couch, or even into the crook of your arm. The question is... why? It seems counter-intuitive to self-defense to do that.
It is not so counter intuitive for a cat to sleep curled up as you suggest. Protecting the head whilst sleeping is not counter-intuitive for safety. If you think about it, it is, perhaps, the safest position to sleep in. The back is exposed and all the soft belly and face is protected. Even the hard skull at the back of the head is a like a little armour.
As opposed to a stretched position, where this picture illustrates well the soft underbelly that the cat is protecting.
Dogs will often sleep rolled up in a ball also, it's good to observe when they do and when they are relaxed enough to lay out stretched, or even have a play stretch.
It's also noteworthy that animals will find the safest place to sleep.