For people using dnf from the console on a daily basis it is redundant to pre-download update RPMs and keep them in the PackageKit own cache (which isn't cleared up anyway - bug 80053).
It's especially problematic for people with (relatively) small root partition on SSD drive. 30GB may be too small after some time (due to gigabytes of completely not needed RPMs).
The another use-case is using a wifi network tethered by a mobile phone with limited data connection.
It would be good to be able to disable update RPMs downloading unless a package update is requested via the PackageKit interface (in the projects configuration, gnome registry, or wherever else).
We moved the upstream bugtracker to GitHub a long time ago. If this issue still affects you please re-create the issue here: https://github.com/hughsie/PackageKit/issues
Sorry for the impersonal message, and fingers crossed your issue no longer happens. Thanks.