Outlook 2007 PST and OST files may be up to 20% larger after the February 24, 2009 Cumulative Update
After you install the February 24, 2009 cumulative update for Outlook 2007 you may see the file size of your .ost and .pst files grow by up to 20%.
The February 24, 2009 cumulative update for Outlook 2007 includes changes related to the general reading and writing characteristics of the .pst/.ost file so that less is demanded from your hard drive to perform the same general operations in Outlook. For instance, the file flushing process was optimized so that it interacts more efficiently with operating system and disk-write caches. There is also a change in the way new data is written to the .pst/.ost so that the data is saved much closer together on the physical disk. As a result, there is less physical work required from the drive, thereby speeding up the time it takes to write the same amount of data to the drive. However, a side-effect of this data writing algorithm is that .pst/.ost files will be about 20% larger than before the February 24, 2009 cumulative update.
If you do not want your .pst/.ost files to use this new process you can disable the feature by adding the following registry data to the Outlook client:
Values: 0 = (the default) new behavior
1 = disable this new behavior
Source -> Article ID: 968413 - Last Review: February 26, 2009 - Revision: 1.0