I'll answer part (3) first (in fact I added it).
3) Describe the processes and criteria used by Debian to add software to the Debian archives.
The software included in Debian basically includes everything that (approximately) surmounts the following hurdles.
Someone is interested in packaging to Debian standards (no easy
accomplishment, at least for complex software, these days).
If the someone is not a Debian developer, then a Debian developer
needs to sponsor the package to get it into the archives. If the somone is a
Debian developer, this step is not needed.
The FTP masters are the gatekeepers to the Debian archives. They have to
accept it. They are in charge of making sure that there the licensing and
packaging standards comply with Debian's standards in the matter,
specifically the [Debian Free Software Guidelines]
(http://en.wikipedia.org/wiki/Debian_Free_Software_Guidelines) and Debian
Policy. Packages can and do get
rejected at this step, sometimes for no good reason.
Subject to these restrictions, any software can be packaged, as long as it runs on Debian, of course. These restrictions significantly slow down how much new software arrives in Debian, but there is still a lot of new stuff coming in. Here is the Debian NEW queue.
The number of packages is going up, probably because interest in Free Software since the birth of Debian has been increasing steadly, more or less monotonically. Therefore the number of people involved in Debian has been increasing. And finally, the amount of free software in the world has also been increasing, though much of it is unmaintained or poorly maintained, because most Free Software projects have a single developer, and in most cases when he or she stops working on it, the project eventually dies. Note that Debian removes packages from testing/unstable all the time, but mostly when there are serious bugs and nobody steps up to fix them.
1) Why did the LoC from the Debian source grew from 55m to over 300m in ~9 years?
I think goldilocks covered that well.
2) Why is there such a big difference to other OS?
The term other OS is ambigious. Roughly speaking, operating systems come in two varieties, proprietary and free. Proprietary operating systems include the various Microsoft Windows OS's and Apple's OS X, but there are many other lesser known ones. These operating systems by default only include a basic set of utilities. While free software may be available for these platforms at most only a small subset of this is available directly from the vendor. There is of course much proprietary software available. In general proprietary vendors like to charge for their software, so don't want to encourage users to use free software.
The free operating systems include the best known, the various operating systems that run on top of the Linux kernels (popularly known as Linux distributions), the various *BSDs like OpenBSD and FreeBSD, and over hybrids variants like the various Debian OS projects that run on top of kernels other than Linux; e.g. Debian GNU/Hurd and Debian GNU/kFreeBSD.
The Linux distributions are mostly similar in the software they offer, though they do vary in the amount of software that is available directly from the vendor. The community distributions like Debian and Fedora have very large amounts of software available. Debian has possibly the largest amounts of software. The more commercial distributions have a smaller selection of software.
The *BSDs also have a large amount of software available, comparable to the Linux distributions, though not quite as extensive. Much free software is sufficiently portable to run across the free Unix-like systems like the Linux distributions and the *BSDs.
In summary, Debian, Fedora, Gentoo and FreeBSD, for example, distribute quite similar amounts of software, so Debian is not particularly unusual in this respect, though it is likely that Debian distributes the largest amount of software of any operating system, though I have not crunched the numbers to find out for sure.