I do not value head size and bite strength too much when it comes to true working terriers.
Yes, I want a dog with teeth, and it does not hurt if the dog knows when to use them, but in truth 99 percent of the work of a working terrier has nothing to do with bite strength; it has to do with nose, experience, chest size, brains, discretion and, above all else, VOICE.
Large heads are a show-ring affectation; a value embraced by theorists. Yes, you need a dog with teeth, but almost any jaw in a dog weighing 10 pounds or more will have a decent set of teeth in it.
The problem with a large head is that it tends to come with a large chest; a morphologically-linked combination because the larger chest is needed to support the larger head.
The problem with having a large chest on a working terrier is that when the dog gets underground it cannot maneuver with the speed and agility needed -- if it can get underground at all.
People who claim that a dog can "get there if it has the desire" are talking nonsense; a dog cannot excavate the 30-foot length of a den pipe, not can it dig out a narrow constriction point lined with rock and root. The true digger, unlike the ring-side theorist or once-a-year digger, knows that most dens are not made of potting soil and corn stalks!
An over-large dog will too often jam itself into a pipe like a cork in a bottle. In such a situation, the dog may be unable to move left or right, and may only be able to move backward with a great deal of difficulty. A terrier in such a situation, with formidable quarry right in front of it, is more likely to sustain unnecessary damage than if it were a smaller dog with a weaker head but a decent voice that it actually used.
Bottom line: Give me the smaller dog with ready voice and brains enough to avoid wreckage. It is an animal that can be dug to week after week.
The over-large mute dogs with the bully heads covered with scars are not flashing a "red badge of courage." Too often they are simply saying they were too big for the pipe and too mute to use any tool but teeth. Seen in that light, scars are not a mark of success, but a mark of failure.