Nor-Cal Chickens
Songster
Welcome to BYC! First things that come to my mind are differences in feed and potentially in water as well. From there you need to see what else are you doing differently. More room to move around, exposure to sun and climate conditions as well?
Glad you joined.
That's an incredibly large flock! I don't have anything to add beyond what others have said but welcome to BYC! 
