There may be physiological differences between women in regards to the amount of glandular tissue and effectiveness of hormone production, but there are other environmental factors as well. Plus, these things can change - women grow more glandular tissue with subsequent pregnancies, for eg.
Frequency and effectiveness of milk removal in the early weeks impacts the ability of your breasts to respond to the hormones necessary to produce milk as prolactin receptors are laid down at this time.
It's a good question! And it would be very good to be able to answer these things definitively. Let's hope things will be easier for our girls.