Is the poo dark green or bright green? Dark green is ok but bright green is the hind-foremilk imbalance.
Drop in qty could be constipation. Is your baby in discomfort or have a loss in appetite? If so, it could be constipation. You might want to bring your baby to a doctor if that's the case.
I have friends whose babies suddenly change poo pattern but it's not constipation.