Wiki says this, although it looks like BS to me, I never read this in the manga:
"Although Tobirama was the technique's creator, his usage had two flaws: the reincarnated souls were not as powerful as they were while alive and he could only control so many at a time.[4] Orochimaru is stated to have mitigated both flaws somewhat, but it is Kabuto who entirely eliminates them, perfecting the Impure World Reincarnation to such a degree that the reincarnations are practically as they were during life and he can control as many at a time as he wishes to, resources permitting."