There's a number of people who believe the head of the Great Sphinx was actually recut into the head of a pharoah. The source linked suggests that the current head is too detailed for the age of the Sphinx (the original head would be very weathered, etc.), hence it would have to have been recut - which in turn leads to it looking too small.

One (of a number of) sources: http://www.robertschoch.com/sphinxcontent.html