Hi Steve

Is this only on a machine reboot or when the service is stopped and
re-started?

When SQL Server is started it will look at each database and roll
forward/roll back the transactions that are uncommitted in the log in the
same way as a database restore would. See the topic "Understanding How
Restore and Recovery of Backups Work in SQL Server" for more information
about these.

Do you have autoshrink set on for the database, if so you may want to switch
it off as this can lead to file fragmentation which will affect performance.

What does the SQL Server log say?

How have you monitored the file size?

Thanks

John