Something really strange (in my opinion) about breastfed babies is that once they are a week or so old they are drinking as much breastmilk as they ever will. Breastmilk changes in composition to give more calories etc so it gets denser/richer rather than them needing more. It is one of the things that makes it a perfect food, but it is totally counter-intuitive. The only time a baby actually needs more is temporarily during a growth spurt. Pretty amazing huh.