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Like I said, his own chakra has been enhanced since birth. Another requirement of SM is a strong body which might be thanks to him being a Uzamaki, but there is no proof that it is.
Like I said, go back and reread the bit after he was first shown in the 4 tailed form, his talk with Yamato when he gives him a hard time for using the 9 tails chakra.
He tell him that his own chakra is what keeps the 9 tails at bay.
Also during Kushina's flashback, the reason she was chosen to be a Jinchuriki is because she had powerful and a large amount of chakra.