I've always read that breastfed babies gain weight more slowly than formula fed babies, but by the time they are a year old, there is no significant weight difference. I think most babies lose weight at first after birth, don't they?

Maybe the weight gain charts are based on bottle fed infants? Heck, I'd also want to know whether formula companies produce those baby weight gain charts, in some sneaky plot to discourage breastfeeding.
My two daughters were both totally breast fed for their first eight months, and one was a slender baby and the other was chubbier...go figure! They both turned into perfect, healthy, and delightful children.

I think that the more you supplement a breastfed baby with formula, the less interest they have in breastfeeding. Formula is easy for the baby to extract from the bottle- it tends to gush out at the slightest suck- breast milk is a bit harder work for them. I also suspect formula is more filling.