Most archeologists assume the original Sphinx had a lion-face when build, in accordance with the Sphinx' body.
Due to wind, sand, etc., the lion's face eroded during time.
At a certain moment, it was decided to sculpture the eroded face into the face of the then living Pharao. That would be the reason why in reality, the sphinx' head is relatively much smaller in accordance with the body.
The Sphinx is generally made of very soft stone and is prone to much more erosion then other contemporary Egyptian constructions. The body was reinforced by later Egyptian dynasties with other stone-material several times, and even the Romans did some restructuring.