Just a quick public service announcement:
Delta rpms (The much smaller files that contain just the changes from one package version to another) are currently not working for Fedora 26 (all other releases should be working fine).
They actually were not fully working before, but the problem wasn’t detected fully until a few weeks after the release. The issue is that we now push out a bunch of alternative arch builds in the same updates as before and we place those in another location on the master mirrors. This means that mash and bodhi (The things that make the updates repos/updates) need to know where to look for the older package rpms in order to make delta rpms against the new ones, but currently we just pass it one location for everything.
We hope to fix this soon, but if you don’t see delta rpms in F26 (yet) this is why.
EDITED: As of 2016-09-01 we have F26 delta rpms fixed. Everything should be back to normal. Thanks for your patience.