Why is fresh restore of backup larger than production database file?
It is a rare case, but you might run into a situation when the freshly restored backup creates a database file larger than the original database from which the backup was made.
This might come as a surprise, especially if you did a backup/restore cycle in order to reduce the database size.
The only known reason (so far) for this is the change in database on disk structure (for more info on ODS see FAQ #117). You might have a database created with earlier versions of Firebird and when you back it up and restore it, the newly restored backup uses the default ODS used by newer version of Firebird you are currently using. Since Firebird improves all the time, new stuff is added to the system tables (metadata) and thus it requires more space on disk.