It’s the muscles surrounding the jaws. The shape of the head is determined by the shape of the skull.
Here’s a boa:
Here’s a boa skull. Note the structure of the jaws, particularly the back end where sit the the two joints that control the gape of the snake (snakes have five mobile and one immobile joint within their skulls). See the boxy, rounded shape those two joints make?
Here are the two overlaid:
Now imagine the muscles needed to control those joints. Depending on the species/size of the snake and the prey they feed on, they’ll need more or less muscle mass to operate their jaws.
See those big fleshy white things in the corners of the mouth? That’s all muscle that, when the mouth is closed, occupies the space in the skull that people like to squish.
That, is headboob.