What I don't know could fill a warehouse means you don't know as much as you think you know.
What you don't know can fill volumes means that you don't know much about what you're talking about.
The same is true for the saying what you don't know could fill a book or fill a library.
What you don't know could fill a book means that you're not very knowledgeable and don't know much about what you're talking about.
What you don't know could fill a book means the person does not know a lot.
Basically when someone tells you that what you don't know could fill a book means that you're dumb and it's a more polite way of saying your dumb.
Another way it is sometimes said is what you don't know could fill a warehouse.
Or What you don't know could fill a library.
It means that you don't know a whole lot about what you're talking about or thinking about even though you may think you do know a lot about the things you think you know about.
My Grandma used to tell my Grandpa that he could fill a library with all the things he does not know.
It was just a fun way of my Grandma and Grandpa arguing in a good way.
They loved each other dearly although they argued every once in awhile and when Grandma died Grandpa said he missed her arguments with him.
He said that if he gets to heaven and she's there as well he will argue again and have fun.
"Do you have a library?"
"What does someone as smart as you need a library for?"
"My lady, they could fill libraries with things you don't know - in fact, they do, they call them libraries. And in my great knowledge and wisdom, I suspect I'll need one."
She narrowed her eyes at me.
"Yes, we have one. Right this way."