Either her head is too small, or her neck is too big/long.

keonas said:

Either her head is too small, or her neck is too big/long.

Well the background lines curve, indicating it might be a field-of-view illusion (breasts appear magnified, center of the fov. Head appears tiny, edge of fov.)

RedVonLewd said:

Well the background lines curve, indicating it might be a field-of-view illusion (breasts appear magnified, center of the fov. Head appears tiny, edge of fov.)

No, the backgrounds have straight lines.
Yes, you can see those 2 effect arcs over her head but, if you look at the line of stained windows, you'll see that they're distancing regularly, in a straight line.