Is a SQL Server transaction log backup the same size as the online transaction log file itself? (4369038)

The online transaction log must have enough information to rollback active transactions, so some space is reserved for eventual rollbacks. If a rollback occurs, SQL Server doesn’t want to expand the online transaction log because if the expanding fails, the SQL Server database can become inconsistent or go into the Suspect mode. That’s why the online transaction log has some reserved space and is usually bigger than the SQL Server transaction log backup

Moreover, a transaction log backup contains only the transactions made after the last transaction log backup. If the online transaction log contains the transactions that have already been backed up, they will not be present in the new SQL Server transaction log backup, therefore the transaction log backup will be smaller for that amount of space