Bite force is not a structural issue alone. Head type doesn't depend upon skull alone or muscle alone. For example, a bear appears to have a pronounced stop when viewed in the flesh, but when viewed in skeleton most bears do not have such a stop. The obtained that appearance not by bone...but by muscle. In addition, bite force is HUGELY contributed to by WILL TO BITE HARD. These are things we don't see when we look at the flesh. You don't get performance by using show breeding methods. Instead, you have to select from dogs that display ability.