I believe
Naruto only got the Kyuubi's soul and body sealed into him while his twin sister had all the chakura.

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Naruto sat in his room, overlooking the personal training ground of the Namikaze family; his so called 'parents' were out on the field with his younger twin sister Azar. 'It's not fair, she gets everything!' Naruto thought to himself, 'They even forgot they had a second child, how do you forget?' It was true, both Minato and Kushina had forgotten their eldest child on more than one occasion.
―Flashback 1―
Minito sat at the head of the table, his wife next to him on his left, his daughter on his right; it was their fourth Birthday-party, but everyone seemed focused on the three that almost no one saw the elder twin slip away, his head hung low at the so called birthday present his parents got his sister. It was a black hilt katana that had the crest of the Namikaze clan on the bottom, along with this was a golden dragon that was wrapped around the top of the guard. A blue diamond pattern went down the scabbard before stopping at the Uzumaki clan symbol.

You see the blade had been made from the clan head swords of clans, the blade and scabbard were from the Uzumaki clan sword, the originals handle had been cracked in the fall of the Uzu-village. The Namikaze swords blade had been snapped in half during the great third Shin-obi war. To add insult to injury Minito himself had promised Naruto the Namikaze sword so he could repair it when he came of age.
-Flashback1 end-

―Flashback2―
Minito sat in his home office, going over the small amounts of paperwork he had to do, when his daughter ran into his office jumped over the desk and onto her father's lap, "Daddy can we go get some ramen?" little Azar asked him. Minato didn't even give it a second though "Sure honey, let's go get your mother." Had Minito remembered his son, he would or remembered that he said he would spend the rest of the day with him after he finished his work.
-Flashback2end-

The two "Parents" had paid so little attention to him they didn't even see the scars that lined his body under his hoodie, or when he came home with a limp from the villagers. Naruto's body couldn't even recognize the sense of touch on over 73% of his body from all the nerve damage that was slowly resisting itself. It was by the devil's luck or Kami and/or Yami had finally taken pity or him, that Naruto gained a teacher, a new mother, a friend, and even a sister (For now).
As he continued to look out the window at the three he gave a snort before shutting the curtains. Naruto slips off of the bead he had been standing on and walks over to the closet, and slides it open to show his small wardrobe, all of it black, along with a large stack of books on sealing and a small sowing set that set atop a book label "Ninja Clothes: Repair and Make" in big bold letters. It was just one of the simplistic book he had in his possession, it went over how to make the most of "Cloth Armor" or Ninja clothes, or just how to make them.

The book had been a gift to Azar from their grandmother, but as soon as their grandmother left the house Azar tossed it in the trash saying "Why would I need to learn that, I'll just buy new clothes" and walked out of the room. Naruto had seen the book and realized two things, 1. His clothes were getting too small, 2. This could solve that problem. After reading the introduction to the book Naruto had gone out to the Uchiha shopping district and gotten the supplies the book had said he would need a surplus of.

Naruto snapped out of memory lane, something had been bothering him since he had learned to mold Chakura; two seals would always appear over his gut and chest when he did so. From what he could find in the sealing books was that the one over his gut was an advances storage seal, and the one on his chest was an old seal that was used to hold souls in the swords of samurai, or some other form of weaponry. Now Naruto was smart for his age, extremely so he could probably out think any Nara clan member, and he had realized that the soul seal most likely contained the soul of the Demon Lord Kyuubi no Yoko, now he was just trying to find a way to communicate with it.

Slipping a large book out from under the top few Naruto flipped it open to the bookmark he had left last time he had taken notes from it. 'It says here "For one to connect to your chosen soul weapon you must be able to reach inner peace, for the first encounter. Many samurai have many different ways to do so, some find it the heart of war, others have been successful by using meditation as a way to reach the soul, this will open a passage, or bridge for each to communicate and work."

Naruto nodded after reading the passage, it made sense to him, for him to open to a new soul, he first needed to make peace in his, a path for each to walk. Sitting down on his floor he went into a lotus meditational position and concentrated on his heart beating. He sat there for about an hour, and was going to give up when he heard water dripping; he slowly opened his eyes and gasped at what he saw.

He was in a cave of some sort, not the kind that goes along the line of dark, damp, cold, scary; but more along the lines of Amazing, stunning, or for a lack of any other words Beautiful. Lights of so many different colors showing through the water from the floor, stone archways held up the ceiling, and wood platforms connected stone platforms together. Glowing green moss was everywhere; from the water's edge, to the stone platforms, and even over some off the wood walkways.

It took a moment for it all to sink in before Naruto heard soft whimpering from down the stone and wood pathways. Being the person he was Naruto moved quickly but silently down the path, and soon found himself in front of a huge bronze gate. What he saw shocked him.

There before him was a beautiful red head woman, her hair spreading down, over her back body, and pooled around her crouched form. She was holding herself her body shaking from sobs. Nine tails draped over her legs and floor, Naruto could only say one thing before passing out "Kyuubi-San?"