limiting sql server backups inside .bak file
The easiest solution would be using a maintenance plan task to clean up old backup files. However, you could consider scripting your way out of this, using something like the following script:


--====================================================================
CREATE PROCEDURE [dbo].[usp_DeleteOldBackupFiles]
@path nvarchar(256),
@extention nvarchar(10),
@age_hrs int
AS
BEGIN
SET NOCOUNT ON;

DECLARE @DeleteDate nvarchar(50)
DECLARE @DeleteDateTime datetime

SET @DeleteDateTime = DateAdd(hh, -@age_hrs, GetDate())

SET @DeleteDate = (Select Replace(Convert(nvarchar, @DeleteDateTime, 111), '/', '-') + 'T' + Convert(nvarchar, @DeleteDateTime, 108))

EXECUTE master.dbo.xp_delete_file 0, @path, @extention, @DeleteDate, 1
END
--============================
-- xp_delete_file information
--============================
-- xp_delete_file actually checks the file header to see what type of file it is and will only delete certain types such
-- as database and log backups. I suppose they expanded this to certain types of log files as well but as you say this is
-- not documented by MS. Just be aware that it will not delete just any file type

-- First argument is:
-- 0 - specifies a backup file
-- 1 - specifies a report file
-- (I'm not sure what the difference between a "backup file" and a "report file" is, since you specify the extension of files
-- you're deleting with the third argument.)
--
-- Fifth argument is whether to delete recursively.
-- 0 - don't delete recursively (default)
-- 1 - delete files in sub directories
--====================================================================



Credit: http://www.sqlservercentral.com/Forums/FindPost1034057.aspx

There's also xp_delete_file:

master.dbo.xp_delete_file