Always best to post a .blend when posting a problem like this, without it, one can only guess…
I’ve seen this problem once before and that time it was caused by the scale of the armature as an object. Think that maybe your problem as well, the screenshots you posted show the head bone to have a scale of .273. I doubt you scaled the head bone down that much for the screenshot, so that’s where I would start looking. In object mode with the armature selected, what is the scale set to? It should be at 1 for x, y, & z. To reset the scale, ctrl-a -> apply scale. If that doesn’t fix the problem, upload the .blend…