These DVDs contain all the binary packages in main. See, for example
Where is the CD image with non-free?, which explains why non-free software is not included. For the purposes of this question, we can consider both the non-free and contrib section of the archives to be non-free software. The contrib section corresponds to software that is itself free, but depends on software that is non-free. Therefore the software in the contrib section is not included, since it does not make sense to include it without the software in the non-free section.
The size of the DVD images may appear to be (relatively) small, but bear in mind that Debian binary packages are compressed. Historically gzip has been used, but more recently more efficient compression formats like xz have begun to be used as well. Therefore, these packages will take up much more space once installed to disk.
For the record, I'm copying the text of that FAQ below.
Where is the CD image with non-free?
Debian has a quite strict view with regard to the licenses of
software: Only software that is Free in the sense of the Debian Free
Software Guidelines is allowed into the actual distribution. All the
other, non-free software (for example, software for which source code
is not available) is not supported officially.
The official CDs may freely be used, copied and sold by anyone
anywhere in the world. Packages of the non-free category have
restrictions that conflict with this, so these packages are not placed
on the official CDs.
Sometimes, someone is kind enough to create unofficial non-free CDs.
If you cannot find any links on this website, you can try asking on
the debian-cd mailing list.
EDIT: It looks like there are indeed 10 DVD images. However, some mirrors only make the first 3 available, on the assumption that those will be enough. I have not been able to find an official source for this, but see Steve's Debian CDs/DVDs page.This says
Wheezy (7.4.0) discs:
Wheezy, the latest stable Debian release...!
Wheezy takes a different number of CDs/DVDs/BDs depending on the
architecture. Check the lists at http://cdimage.debian.org/debian-cd/
to see exactly how many, and which packages fit on which disc(s).
As a guide, amd64 or i386 take 10 DVDs each if you want the whole set,
but it's not likely that you will want them all. For most people, a
set of just the first 2 or 3 DVDs is likely to include all the bits
you'll need. All the source fits on 8 DVDs.
You can find a list of the DVD contents at