I agree with KYF on this.
it does appear that Madara didn't expect to be so powerful, I mean he looked down his shirt and was like, "oh, ooookaaaay that's why."
Also to be brought back in his prime would be without RG. Granted he awoken it before he died, being so old he probably didn't use it much.
look at him since he has been back. Like a kid with a new toy, he could have killed the Kages from jump, but oh no he had to see what he could do.
Just like NOW he has the Juubi, he has to see what it can do, which will be his downfall.