In SQL Server, the transaction log files grow unboundedly until you take some action. To completely discard old transactions, perform the following steps in the database in question:
backup log databaseName to disk='nul:' dbcc shrinkfile('logical log file name', 64)
If you actually want to save the contents of the transaction log, use a better pathname than nul. If you do not know the logical log file name, execute this query:
select * from sys.database_files
In practice, I have found that you may have to run the BACKUP LOG command a few times to get the file to shrink down significantly.