Does a dog's head size determine psi?

I heard the mastiff's bite with 500lbs +, What about the anatolian shepherd, they are built do defend flock's of sheep, and other livestock that they defend. They have skinny heads, sorta like dobermans. They are massive though. Thanks people. I just want to know, I'm kinda confused. Don't answer with you don't have to know, blah blah blah. It's a curious question, I'll delete it afterwards. Chill out.

Size, no, but shape and degree of muscling, yes. Those with shorter muzzles and bigger muscles in the chewing area are going to have more psi than those with longer more elegant snouts. Its a matter of physics. Mastiffs, pit bulls dogs, boxers, English bulldogs, rotties, etc are going to have more psi in their mouths than dobies, collies, shepherds, greyhounds, etc.

There's a youtube video on this, it might interest you.http://www.youtube.com/watch?v=MZm037jPNgc

Yes it does. Bigger muscles need bigger anchors to keep them attached to the bone. Big muscles need a greater area of bone.If you look at animals such as tigers, male adults develop a fin of bone called the saggital crest. This anchors the jaw muscles and increases the strength of the bite.