Às reply to TJ, I do agree that - at first - it does help to first Defrag your system before creating a store.
However, in our case the data stores are moved/created to a completely different volume which was completely empy - just formatted.
After migrating all our data from groupwise and a month worth of production, our database has grown to 81GB... when using the analyze feature of defrag I only see a huge amount of red and next to none of blue lines...
Perhaps this is due to the nature of the Jetbase engine. Personally I like the system of Oracle better where the database has a particular filesize to start with so there simply won't be any fragmentation. (provided ofcause again that the drive was unfragmented to start with.)