Or: memory clumps.
Searching for a small item in a large pile of junk is quite daunting the task. For bigger items it’s a lot easier though, once you find one part of it you can fairly easily pull out the rest. In a sense, this is similar to how associative memory works. One recollection clings to another, and so a clump or chain of knowledge can easily be pulled up in one go.
One could guess that this is also why learning in so many different ways and environments is key. You want to obtain as many distinct memories of the thing you’re studying, allowing for faster recall. Of course, this could also be repetition at play, but the two may go hand in hand.
I’m not neuroscientist though. Just musing.