I do full backups of my database twice a day. And I back up the transaction logs every 30 min. Before last week the transaction log backups were between 1 and 10 MB depending on the number of transactions performed during the log backups. However, last week the size grew to 500 MB and stays there even if very few transaction are performed between log backups. I have tried shrinking and truncating the log file. But it just jumps straight back up to 500MB.