You wrote: 

Diminishing returns.  It comes down to personal tastes, but I find 
that even good fanfics have a limited number of things to say.  If 
everything can be said in 20K it's more interesting than expanding it 
to 500K, IMHO.

I'm surprised by this observation.  Are you saying by this that there 
is no such thing as a good fanfic novel, or simply that most fanfic 
writers are incapable of writing one?

One point: I think it depends on the universe you're writing in.  IMHO, 
RANMA 1/2 is more suited to short stories than to long ones.  On the 
other hand, any GUNDAM story would have to be at least a short novel to 
be worth the effort.

------RM