Was this ever said? I just checked and Rayleigh never said that it grows. I just assumed people started with a set amount of CoC and thats it.
What was stated or shown is:
- Everyone has CoA and CoO, because haki is in every living being, human or not, they just have to find out about it.
- CoA and CoO can be trained.
- Only a few have CoC and it cannot be trained.
- However, what did he mean by train it? By "force of habit" (which comes from practice, or in other words, training) Luffy or anyone else can go from uncontrolled outbursts to targeting specific targets at close range amidst a crowd, nor affecting anyone else. His CoC wasn't stronger there, but Luffy was many times more skillful in its use after the timeskip.
My personal interpretation on CoC is:
Haki is deeply tied to emotions, it's understood to be similar to willpower and tied to it. Just like it can bloom in battle, it can be affected both positively and negatively by emotions. While I do think, like Rayleigh told us, that CoC cannot grow with training, I do believe it can become stronger as a character grows in battle.
That's my belief until proven otherwise by the manga.