Delta RPM

https://fedorahosted.org/spacewalk/wiki/DeltaRpmSupport
http://www.linuxtopia.org/online_books/opensuse_guides/opensuse11.1_reference_guide/sec_rpm.html

Create a drpm:
makedeltarpm oldrpm.rpm newrpm.rpm deltarpm.drpm

Create drpms from a full repo:
createdeltarpms ./repo ./repo/DRPMS

Reconstruct original rpm using on disk installation:
applydeltarpm delta.drpm new.rpm

Reconstruct original rpm using an old rpm:
applydeltarpm -r old.rpm delta.drpm new.rpm

Create a presto repo (and the prestodelta.xml file):
createprestorepo ./repo

Then to finish the deal and link the prestodelta.xml to the repomod.xml file:
modifyrepo ./repodata/prestodelta.xml ./repodata/