Size of the ldf file increasing
Backing up the transaction log will not shrink the physical file.  It will only mark transactions as inactive and potentially mark virtual log file(s) for reuse.  What you're looking to do is actually shrink the file system file, and this can be done through DBCC SHRINKFILE.

Note: shrinking the transaction log, or any other database file for that matter, should not be a routine database task.  Proper sizing will mitigate the necessity for tasks like this.