Anne A.,
I don't think the mom has a different amount of calories in her milk,
it's probably the way she and baby are managing the milk supply and
feedings.  I've met women who had enormous six month old breastfed
babies (petite and average size parents), on questioning these women
described overactive let downs and milk supply self  managed by allowing
fore milk to run off  in a cloth at each feed.

For months these babies were having the mostly higher fat Hind milk
feedings. IMO the way these moms self managed OALD hind / fore milk
imbalance caused  the babies to gain the extra weight.

Ruth Callahan
NY, NY