The present head is not the original head, but is cut down from the original head to about half the proportionate size. In addition, the neck is not smoothly faired into the body anymore but was chopped to fit the present head. The original head was of a lion, there being enough material in the original yarddang outcrop, and when the muzzle either broke off due to weathering, or was cut off to put the human face on, most likely when the body was buried in sand as it was until recently, the proportions of the rest of the body were ignored.
That's the theory. One wonders what secrets the Sahara holds to be discovered, that may relate to a civ which carved the Sphinx originally. There's grist for a great novel in that, somewhere. I have one related to al Shamiya Desert, perhaps I should do one for the Sahara?