I think it really varies a lot and I'm not sure WHAT causes big babies, exactly.
My DS was 9 lbs. 12 oz. and I ate pretty much whatever I wanted to eat during his pregnancy. That included a LOT of ice cream.
He was a forced induction at 41+ weeks and ended up being a c-section birth. He nursed like a champ despite a late start (I didn't see him for 24+ hours after the birth) and the nurses feeding him formula and glucose water bottles in the nursery DESPITE our insistence he receive only breastmilk and room-in. I gained 23 lbs during the pregnancy and lost it all within 2 weeks of the birth.
DD1 was 7 lbs 12 oz. and was born naturally at 41 weeks 6 days. I followed a low carb version of the Brewer Diet religiously. I gained 25 lbs during the pregnancy and lost it within a month of birth. She did have suck issues and has a host of food allergies (I believe in part due to my pregnancy diet).
DD2 was 8 lbs 12 oz. and was born naturally at 39 weeks 5 days. I once again ate pretty much whatever I felt like eating. I did limit my dairy intake and my peanut intake to try and reduce chances of allergies. I gained 26 lbs during the pregnancy and lost it all within a week of the birth. She breastfed great from the start and has no evidence of allergies so far.
For me it seems that diet doesn't necessarily correlate to birthweight for the kids and I tend to gain about the same amount of weight during pregnancy regardless of what I eat. Eating a high protein diet resulted in my lowest birthweight child and the only one with severe allergies.