Does having big babies cause diabetes or does diabetes cause one to have big babies?

Diabetes - big baby: Having diabetes predisposes a woman to have large babies, not the other way around.

Babies adjust to mom: During fetal development the baby will begin to manufacture it's own insulin, the large protein that helps carry glucose into the cells. When mom has diabetes and runs glucose levels that are higher than normal, baby adjusts and makes more Insulin to help store the excess glucose. Baby is not only bigger but its organs get bigger and it's metabolism is altered. Sugary moms cause big babies.