Transaction backup plan and size
I cannot say why your daily transaction log backups are large - it is usually down to user activity on the database and SQL Agent maintenance jobs such as ones that use DBCC CHECKDB and re-indexing.
WITH RETAINDAYS will not delete your previous backups, it will prevent overwrites of the backup file when using the INIT option.  You will still need to run a job to clear out the old backup files.
Here's one that Ola Hallengren's scripts add :-
    DECLARE @CleanupDate datetime
    SET @CleanupDate = DATEADD(dd,-28,GETDATE())
    EXECUTE dbo.sp_purge_jobhistory @oldest_date = @CleanupDate