What is the Average bra size for 13?

My mom and all her family have big boobs from c-d. My mom said that she was a c cup in my grade and I'm only a 34 a. I feel like really small cause most girls in my grade are c's and b's. Also my dads side of the family for the girls are d's. But one of my moms aunts are smaller then the rest and she said i could have taken after her. And i want to know the average size for a 13 year old girl.