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.
Most people will not have noticed the difference as many corporate clients use Exchange & Outlook so never see the OST files but those with PST files may have. For what it’s worth with the cost of storage being so low I would put up with the storage increase for the performance gain.