I too, have a fairly small female that bites much harder than all my larger dogs. Size and weight could be considered, but without a 'commitment', don't really mean alot, I think. Here's a few notes I took on some studies I found which support the idea that strong grip is a genetic trait...
Studies in primate bite muscles...
While studying muscular dystrophy in humans, scientists tracked down a gene that expresses the powerful bite muscle. Bite muscle is the most powerful of the jaw closing muscles and completely encloses the skull in all non-human primates. The same gene was also active in a sample of human bite muscle, but the human gene has two missing base pairs in a key region of the gene. The muscle fibres in humans are smaller than those in other primates and exert less force on the skull, allowing it to grow and expand. Detailed genetic analysis suggests the human mutation occurred approximately 2.4 million years ago, soon after emerged the earliest known members of the genus **** - with smaller jaws, and larger brains.
Studies in humand hand grip strength...
Studies of hand grip strength in 1,757 Danish twin pairs aged 4596 years, found that this phenotype has a heritability of 4855%. This phenotype is correlated with muscular power in all age groups and with other muscular groups, and is a suitable phenotype for predicting mid- to late-life physical functioning.
I think you need a good combination of physical Capability, plus a strong nerved Commitment, both of which are largely genetically dependant.