YES - my baby is big too! I have a ds who is just over 5.5 months and weights about 18 lbs! He was an average baby at birth, 8lbs even. He's just an eater!
I breastfeed too and, recently, have been wondering if I over feed him. He will always open his mouth for food and nurses every 2.5-3 hours. I was trying to get him more on a 3-3.5 hour schedule but his night sleep is horrible so I started nursing him more during the day in hopes of getting more calories in him. It's working, sort of. He is sleeping a little better, but not great.
Anyways, I did start him on cereal about a week ago and he only gets it 1 time/day and only about 1 tbsp. I always nurse him an hour before the cereal so that he won't over eat. Honestly, he does eat it all b/c it's only a tbsp, though.
Is she moving yet? My doctor said once he's crawling, pulling up, etc, that we'll probably won't see a whole lot of weight gain thru a year. He'll just get longer. Some babies just love to eat and are big!