>Yes it is a big size because the corresponding data is not that much. After backup and restore, the size decreased to 4.7Mb.
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>Making backup and restore is not pratical for everyday running databases. How to erase the unused bytes?

The way Firebird typically behaves, is to first grow a bit, then reach a level where it stabilizes and more or less stops growing. If it doesn't stop growing, then this indicates that there is a problem with the transaction handling in one or more of your programs. When you update or delete a record, then Firebird stores a new version of that record. Old versions can be replaced, but not until all transactions that can be interested in the old version commits or rolls back. I don't know Firebird internals, but a database that grows to 10 times its original size (although 48Mb is still small) makes me suspect there's still one or more old transaction(s) kept open (you need proper commits or rollbacks everywhere, CommitRetaining isn't a proper commit).

HTH,
Set