Time Machine may not be too bright

[struggle4progress]

(1) Ran TM for the first time on a notebook: took a while with the usual tedious preparation for copying 25GB+ &c
(2) Realized I can't install Safari4 without Security Update 2009-001
(3) Installed the Security Update (< 50MB) & Safari4 (~ 13MB)
(4) The 2nd TM run is taking as much time and space as the first: TM redid the usual tedious preparation for a first-time back-up and now it's copying another 25GB+ to the backup drive

I wonder if Security Update 2009-001 only changes a few files but forces a rebuild of OS with new dates for a lot of other files that weren't actually touched