In its defence to the copyright action by an angry horde of movie producers, iiNet has argued that because BitTorrent files are broken up into small chunks and the small chunks transferred separately, customers do not transfer "substantial portions" of the files and so they do not breach copyright.
This argument misunderstands the meaning of the word "substantial" in the phrase "substantial portions". "Substantial" in that phrase refers to substance, not size. If the portion contains any of the "substance" of the protected expression, it is a "substantial portion". The amount of material copied is not relevant at all.
The amount is relevant to the "fair dealing" defences, but that has nothing to do with the "substantial portion" question.
Since BitTorrent users will, over the course of their session, copy the entire file, there is no doubt that some of the portions copied will contain substance of the protected expression.