Naruto's chakra capacity...
Naruto has 2 seals to hold back the 9-Tails.
#1. Physical Spiral on his stomach.
#2. The gate inside him, with the small piece of paper on it.
*** NOTE that ALL of the 9-Tails chakra, both ying and yang, is located behind the gate. The 9-Tails CANNOT come out, only its chakra can "leak" out.... this is because of Seal #1.
How #1 Works:
Have you ever seen the inside of a clock? It uses a spring (coil similar to Naruto's stomach seal) for its timing device. In order to keep the clock running, you need to wind it up every so often... Just like Jiraiya said about Naruto... his seal opens/unwinds over time... This is what the Key-Scroll is used for.
What this means is that the stronger Naruto gets, the more the seal unwinds... Naruto's own chakra pushes the seal open. If the seal is unwound artificially, ie using the Key-Scroll like Jiraiya did... The 9-Tails chakra FLOODS into the empty space, that Naruto is unable to fill with his own chakra. AND Naruto loses control of his body.
How #2 Works:
The 9-Tails is always sealed inside of Naruto, unless Naruto pulls the seal off the door (Or Sasuke)...ALL OF THE 9-TAILS is in here.
COMPARISSON.... Look at Bleach, and the BOUNTS' DOLLS.... remember when the big guy with the Metal Doll did a flashback, and showed the kid he was teaching how to be a bount. The kid went off and tried to make his doll by himself. He was not powerful enough to control the doll, so the doll overpowered him, and consumed them both... There is no bount that is only able to use "portions of the doll" until he is strong enough to use it all....
This is exactly what the 4th did for Naruto though. Something no other Jinchuriki had...