It is not the length of the neck that is wrong, in Fuhrman's drawing. Rather it is the position of the exit wound which was in the nape of the neck to the left of JFK's tie knot. Your profile photo shows that Fuhrman's placement of the exit point of the bullet is nowhere near the actual exit wound on JFK (which is below the bottom of the photo where his tie knot would be).
So Dox drew the throat exit wound a little high. But maybe she drew Kennedy neck as it was in life, not as it's seen in the Left Profile autopsy picture.
The fold of fat beneath the point of Kennedy's chin in the autopsy photo is stretched and angled, while in the life photo, the fat under Kennedy's chin is nearly level. This would mean that in life, the exit wound was a little higher than what's seen in the autopsy photo.