Hi Stefano,

To decrease database size you need to make backup/restore.

Then, to keep its growth under control, you need to perform the
following maintenance procedure:

1) stop Firebird server and then start it immediately (less than 3 sec
downtime).
2) run
gfix -sweep -user SYSDBA -pass masterkey <database_path>

that's all.

Regards,
Alexey Kovyazin
IBSurgeon