You know Sasuke ran out of chakra when he hit Kirin right, that why oro took over, Sasuke barely had enough chakra to stay alive. Itachi after kirin had enough chakra to hold susanoo for a while, I´m not sure but I think Itachi busted out more Jutsu then Sasuke as well. Anyway I think that fight made it clear that Itachi´s chakra supplies are greater then Sasukes. Question is, by how much.