Background Noise,
The updates vary according to your Mac. My Macbook update was 442Mb, yet on the iMac was only 280Mb.
There's another button in Disk Utility marked "Verify Disk". Try this. If it flags up anything, move onto the button beneath marked "Repair Disk". I've only had to use this once when an update went awry. It repairs the file structure of the disk itself, rather than "Repair Permissions" which deals with the applications themselves and associated files.
Applemacster.