And when I'm reading fanfic in fandoms I don't know, I find the short fic harder to grasp than the long.

Funny, but I don't always find that to be the case. Sometimes with short, intensely emotional pieces, even without the full resonance of canon, I get enough of the human flavor that I can enjoy them. And sometimes with the longer works, without that canon base, my attention drifts. But that could be because I don't have much of an attention span these days. *g*