I think by Lockjaw you actually mean TMD Temporomandibular Disorder. This is often caused by clenching your teeth or grinding your teeth a lot.
Common symptoms may be:
Pain or tenderness in the face, jaw joint area, neck and shoulders, and in or around the ear when you chew, speak, or open your mouth wide
Limited ability to open the mouth very wide
Jaws that get "stuck" or "lock" in the open- or closed-mouth position
Clicking, popping, or grating sounds in the jaw joint when opening or closing the mouth (which may or may not be accompanied by pain)
A tired feeling in the face
Difficulty chewing or a sudden uncomfortable bite – as if the upper and lower teeth are not fitting together properly
Swelling on the side of the face
I myself grind my teeth and clench my jaw a lot when I sleep so I am prone to getting a sore jaw. Its not necessarily the girth, but the repetitive action of keeping my mouth open for a long period of time. I wear a nightguard which helps with the teeth grinding at night, but I still get jaw pain when giving head. I really don't know a good solution for it other than switching things up. Problem with that is when a guy is close to finishing that can throw everything off :(
LOL .. the first time I did it, my jaw hurt .. but that's only because I never did it before and wasn't used to it.
But other than that, no.
I imagine an average sized one would prevent a girl from her jaw hurting, but I think it all has to do with the position .. no matter if it's average or big, if I'm in an awkward position for a long period of time, there will be an annoying pain
for me, the amount of time I'm doing it doesn't matter ..
I guess I'm one of the lucky ones that doesn't get 'lock jaw' lol :-p
Lockjaw, also called tetanus, is caused by a bacterium called Clostridium tetani. One of the symptoms of lockjaw is a tightening of the jaw muscles, which can gradually cause the jaw to be difficult to manipulate, affecting speech and swallowing. Untreated, lockjaw can cause death, though this is fairly infrequent due to vaccinations for tetanus, and the practice of re-inoculating people with cuts from old fences or dirty metal objects.
Lockjaw was thought to be caused by cuts obtained around horses, which are frequent victims of tetanus. Actually, Clostridium tetani lives abundantly in soil all over the world. It can survive for about 40 years, so infection is certainly possible as a result of a cut from an old nail.
Ya can't get Lockjaw from giving head, unles the guy was dragging his d*** in the dirt beforehand.