On Tue Dec 16 2014 at 9:09:59 PM Chris Murphy <lists@colorremedies.com> wrote:
Fresh installation of Fedora 21 Workstation, accepting defaults, I
then reboot and notice the following contents of /var/cache, filtering
out things not relevant for this discussion (which also happen to not
change between the three states).

Starting point right after installation:

[root@localhost cache]# du -sh *
16K dnf
85M PackageKit
4.0K yum


Login, wait for ~ 1/2 hour:

[root@localhost cache]# du -sh *
94M dnf
446M PackageKit
4.0K yum

That's 455MB of silently downloaded data, by default. Upon doing a yum
upgrade, but rejecting the actual upgrade:

[root@localhost cache]# du -sh *
94M dnf
446M PackageKit
137M yum

There  is some problems in your numbes, not everything in cache is downloaded. In the dnf case only a repomd.xml (one for each repo, <5K) is download every time metadata cache is expired, and only changed metadata will be downloaded (repodata/*.xml.gz), the Fedora repo dont change, so it is only downloaded once, Updates is composed once every day, so it will only be downloaded one a day.
Rest of the content of cache/dnf is generated from the unpacked metadata, so the size of the directory don't tell any thing about how much metadata is downloaded and how often.

Tim