Most large babies burn more energy than average babies. This is mainly because their metabolism is just a little higher. Why is this? Gestational diabetes is one cause. Also, simple genetic differences in people will cause a newborn to be a little larger, and whatever was causing this baby to be larger, will also cause the metabolism to be just a little higher. However, in the first few days of life, they are still getting the same amount of nutrition from mom as a smaller baby would, so they can be prone to low blood sugar.
A baby’s liver is responsible for helping the body utilize energy, so any immaturity of the liver could also cause a baby to have a low blood sugar. An immature liver will also cause a baby to be more jaundiced than normal – so this is probably why your baby had both a low blood sugar, and jaundice severe enough to require extended phototherapy. Bottom line: not much you could have done to predict this. It is considered a minor complication of being a large newborn, and most babies work through it easily with minimal help from the doctors.