That is a big baby. My girls we adopted were tiny - 5lbs 12oz and 6lbs 1oz. So they didn't drink that much. She is likely drinking 14-20 oz a day.
Kelly mom says from 1-6 months most babies drink between 19-30 oz of breastmilk a day.
http://kellymom.com/bf/pumpingmoms/pumping/milkcalc/And that before 1 month it will be less than that. I have read average from 1-6 months is 24 oz a day. But every baby is different.
Weighing her should give you some reassurance, but remember the goal is to be back up to birth weight by 2 weeks, so if she isn't there yet, that is okay. Is she peeing and pooping? Does the poop look "seedy"? It is so hard to trust it, but you have to.
I actually had a baby scale that I used to weigh the baby before and after some feeds to give me an idea of how much she was taking. But really, you have so much milk, I wouldn't worry.