His power wasnt increased, it was just being used by someone who knew how to use it better.
Jug could be 8th right now if he knew how to use his powers. Full potential Jug IS 8th day.
Cyttorak is insanely powerful. Take a look at jugs now, Cyttorak said jugs failed him basically Cyt' wanted to retake his power, but still couldnt take the power the Gem gave Jugs.
He only could take some of it back.