The big box is all knowledge, including the vague 'knowledge of experience' that people talk about in this thread. The box-inside-the-box is verbal/declarative/metaphoric/propositional/philosophical knowledge, that is anything that is fodder for communication in any way.

The metaphor is intended to highlight that people seem to conflate the small box with the big box, leading to confusion about the situation. Inside the metaphor, perhaps this would be people saying "well maybe there are objects inside the box which aren't inside the box at all". Which makes little since if you assume 'inside the box' has a single referent, which it does not.

Edit: I read your link, thanks for that. I can't say I got much of anything out of it, though. I haven't changed my mind, and my epistemic status regarding my own arguments hang changed; which is to say there is likely something subtle I'm not getting about your position and I don't know what it is.