The sizes of software updates can vary for computers running Mac OS X 10.3.4 or later. This is a feature of Software Update, which automatically detects the best update for your computer.
Sometimes, Software Update preferences is able to offer a "smaller-sized Delta" update than you might expect, or smaller than what you might see on other computers installing the same update. The "smaller Delta" updates are offered when some Mac OS X system files can be modified instead of replaced in their entirety.
Not every computer that has Mac OS X 10.3.4 or later can benefit from smaller Software Updates. Why? Sometimes, modifications made to Mac OS X system files, including ones made by third-party products, may require the installation of a full sized version of a Mac OS X software update. Again, you don't have to worry about figuring out which kind of update is best for you, just let Software Update preferences do the work.