@neokakarott: It may seem odd to defend Kishi, but he already implied that the power of the Bijuu isn't equal between them or even linear, but exponential.
Think of it like the numbers of tails are the exponential of a chakra quantity (x). So the Ichibi would have x^1 chakra, the Niibi x^2, and so on. That way, Kurama and Gyuuki would represent 75% of the original Juubi chakra, explaining why the Juubi is pathetically crappy as of now.
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