Wow I think there would be some folks who would be envious of your baby's size!
Do you have relatives on either side who are big-sized too? What did your pediatrician say?
In any case, i dont't think it's possible to overfeed on breast milk, and you're not supplementing with FM so I doubt it's her caloric intake. Babies do feed frequently and it helps with keeping your supply up as well.
What was her birth weight?