Starting with IBM Domino 9.0, DBMT has been introduced. You can use the DataBase MainTenance tool for performing multiple daily/weekly administrative tasks on user’s mail database files. The DBMT tool relieves the administrator of the need to run the Updall task.
When DBMT is executed, it uses the copy-style compact (compact -c ) to compact databases. Compact -c has the disadvantage of fragmenting files and this can have an impact on the performance of your server.

But DBMT behaves a bit different here. DBMT preallocs the .TMP file to the used space size of the .NSF.

I did a test on my server. Here is the result.

Unfortunately, the “old” compact task itself does not yet have the preallocation feature included. I hope, that we will see this feature in a future release …