lots of people are kinda confused because the translators havent done a very good job at explaining it.
the terms in kanji are 貯畜 (chouchiku) which isnt a word on its own, but the 2 kanji together sorta mean "storage". the other one is 放出 (houshutsu) which means release.
The translator chose to use the terms 'potential' for 放出 and 'capacity' for 貯畜. I guess the terms are okay, but wernt explained properly.
Basically think of the body as a water tank. Magic capacity is how much mana you generate and can store. magic potential is your ability to release it to use it to create magic phenomena. In other words, you need good potential to be able to cast powerful magic, while large capacity will give you stamina to keep casting more/longer magic.
Nicole has huge and growing capacity, but very poor ability to release it. and as she grows her mana generation is outpacing her body's ability to store it. Since she cant release it well either that means its overloading her body stunting her growth and causing her to always be weak and fatigued. When her mana exceeds her capacity, she faints as a sort of protection to allow the excess mana to be released from her body.