Microsoft Security Patches
Given that the number of files and total size to download steadily increases I'd say they are cumulative.
This is (parts of) the track record of the 5.1 QSUs (published as 9ARD183777-017 on ABB Library):
2015 APR: Rev AE: 4 files (1900+1900+1900+75 MB)
2015 JAN: Rev AA: 3 files (1900+1900+1770 MB)
2014 SEP: Rev U: 2 files (1900+1100 MB)