I'm not sure why the people who drew the reconstructions have assumed that the faces were flush with the wall masonry.
Looking at alxbates' initial picture, the sculpture on the left already seems to be flush with the masonry, even without a head. On the right, even though the sculpture is relatively more recessed, note how the masonry is curved, as if to accommodate a missing head.
I would suggest an alternative reconstruction where the heads protrude beyond the masonry. In protruding, there would be no constraint by the masonry on proportionate head sizes.