My last post on the subject...Originally Posted by Mizuchi
Once again, you may be right about the concentration aspect. Now, I'm sure no one but Kishimoto can actually tell us the fine science behind it, but I believe that concentration cannot be measured quantitatively. Because of it, the kage bunshins that Naruto produces are possibly weaker physically and have less total chakra than the original, but they all share the same state of mind.
That being said, more chakra output = stronger clones that can produce stronger attacks.