Why do you think my baby is already 9lbs 6oz at 37 wks?

I had a sonogram last week where they informed me of how big our baby girl is already. I'm 5'1" and have gained 70lbs with this pregnancy. I don't have preclamsia or diabetes. He scheduled a c-section for 39wks (in two days) stating the baby is too big to fit through my pelvis. I'm fine with this, but I just wonder why she's so big. I can't help but feel guilty that I gained so much weight and that I did this to her. BUT, my husband is 6'2" and he was a 9lb baby and his brother and sister were 10lb babies. I was only 6lbs. Anyone have know why our baby is so big and will probably be bigger when she's born?