As soon as you take a full backup of the database, and the database is not using the Simple recovery model, SQL Server keeps a complete record of all transactions ever performed on the database. It does this so that in the event of a catastrophic failure where you lose the data file, you can restore to the point of failure by backing up the log and, once you have restored an old data backup, restore the log to replay the lost transactions.
To prevent this building up, you must back up the transaction log. Or, you can break the chain at the current point using the TRUNCATE_ONLY or NO_LOG options of BACKUP LOG.
If you don't need this feature, set the recovery model to Simple.