Something just hit me, isn't the sacrifice used for Edo, the "container" for the technique? I think it is. And, if it is the "container" for the dead person's soul, then the quality of the container would affect the power of the person revived. What I mean to say, is that If the person offered up is weak or has limited chakra, then the revived person will be limited to that amount of Chakra.
If you think about it, it would explain why the 1st and 2nd Hokage were so weak when they were revived. Orochimaru's minions were too weak to support the 1st and 2nd at full power.