At least 3/4 of those 4000 lines is directly copied from Skype, and that's just one file. I don't think that's small enough to not matter; anyway, if it really was so small, it should easily have been rewritten with original code.
No that's a simplistic approach. Both matter, for example one volume of the Encyclopedia Britannica is a very small portion of the overall work, but it cannot be freely copied. Likewise if you write a book a million pages long, copying entire chapters does not suddenly become ok.
It is my understanding that "the relevant copyrighted work" in the Encyclopedia Britannica example is likely to be the article rather than the entire encyclopedia.
