A shorter jaw means the teeth are closer to the muscle, which means it's easier to put more pressure down. Things like crocodiles do get an incredible amount of strength in their bite too. But for any dog the most force is is closest to the head.
Dogs like a bullmastiff bite and hold down big things. Dogs like terriers, that were bred to hunt rats, more instinctively shake their heads really fast, to kill the rat, which is why they tend to tear more while a bigger breed will pierce down deeper. At least that's how I see it.
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