It seems as if Kabuto was drained of all the extra chakra he gained from orochimaru after implanting himself with his cells are his death.
Orochimaru has also been shown to increase in power as all the modifications Kabuto made to himself (e.g. Hozuki clan technique, Uzumaki Life force and Dragon Sage Mode) seem to have vanished from Kabuto's body (normal face, not scaly) and must have gone to orochimaru as they couldn't have just disappeared in the chakra transaction.
This is a graph showing both Orochimaru's and Kabuto's power levels from the time of Oro's death to the time of his revival (Chapter 593).
Kabuto__________
Orochimaru _________
Orochimaru has also been shown to increase in power as all the modifications Kabuto made to himself (e.g. Hozuki clan technique, Uzumaki Life force and Dragon Sage Mode) seem to have vanished from Kabuto's body (normal face, not scaly) and must have gone to orochimaru as they couldn't have just disappeared in the chakra transaction.
This is a graph showing both Orochimaru's and Kabuto's power levels from the time of Oro's death to the time of his revival (Chapter 593).
Kabuto__________
Orochimaru _________
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