Even arguing that borrowing is temporary seems a poor argument at best. Most people only play games or watch movies a small number of times and then rarely pick them up again. And, even so, there is nothing stopping borrowing again.
If the library paradigm is okay, why is the downloading content paradigm not okay?
I think what you're probably thinking of is not library shelves but library checkouts. And I think that you'll find that library checkouts did consist 90% of trashy paperbacks, the text equivalent of pop movies, back when people got that kind of stuff from libraries instead of online.
If you were to judge the contents of a library by the size of the largest items on the shelves (exactly as you have done with torrents), you would come away with the mistaken impression that they consisted primarily of dictionaries and boxed sets of language learning CDs. In fact, these items represent a very small portion of the items in the catalog.
What's being counted as a single item here is not a single bound volume of a chemistry journal, nor the entire archive of Bioconjugate Chemistry, but rather the entire chemistry-journals wing of the library: 539 gibibytes, including 226 different journals. By comparison, the latest five items on http://webcache.googleusercontent.com/search?q=cache:http://... are 3.7MiB, 11.7MiB, 350MiB, 730MiB, and 260MiB; the chemistry-journals library is some 2000 times the size of the median of these and 120 000 times the size of the smallest, which happens to be a two-volume book called "Great Moments in Mathematics".
It turns out that when you have a power-law distribution crossing five orders of magnitude, like the one that characterizes file sizes, rather than the much narrower distribution that characterizes book sizes, you actually can get a useful approximation of the makeup of the total by looking at the makeup of only the largest items. It's surely not an unbiased estimator, but it's still a useful one.
Feel free to invest the work to do a better approximation.
Unless one is loading a moving van or trying to estimate the number of shelves required to store it, one characterizes the contents of a library by the items in the catalog and their subject matter, not by the volume they consume. You don't go in and ask for "a cubic foot of books" any more than you torrent "a megabyte of music".
The latter is more interesting: does one need to be capable of suffering a loss of an object in order to share it? Does access have to have a bandwidth limit for sharing to be reasonable? An interesting example are ebooks at libraries. My university offers a number of these. They are not able to be stolen like regular books and they have no limit on simultaneous borrowers. Even if there were some artificially imposed limit, what purpose would it serve? Do extra restrictions on use make a product more valuable? Does it create more profit for content creators?
Is this really the case. Based on my, admittedly very limited, outsider view of the scene most distributors get their copy via review copies sent to the press or it's an inside job by someone working for one of the companies in the production chain. Having a pirate copy up before something is available in the store is a big deal in the pirate scene and that rather precludes buying a copy.