A baby's birth weight oftentimes has NOTHING to do with future size. Genetics play a big part in FUTURE size, and if the parents are obese, the child may end up being that way also (genetic predisposition as well as environmental factors).
You really shouldn't prejudge this; it could be that the baby is LGA/IDM-large for gestational age and infant of diabetic mother.
Not prejudging, just saying that if I was a betting man, I wouldn't bet against those odds.