SQL 2005 Maintenance Plans

One of my SQL servers runs a maintenance plan every 4 hours, the first 3 are approx 17m the 4 bak file is 248,456M this repeats every day.
Also does not cleanup the older files. Is set to delete any older than 7 days.
Microsoft SQL Server Management Studio
Why is the size of the bak file increasing so dramatically would be the question!!! 9.00.1399.00
Microsoft Analysis Services Client Tools 2005.090.1399.00
Microsoft Data Access Components (MDAC) 2000.086.3959.00 (srv03_sp2_rtm.070216-1710)
Microsoft MSXML 2.6 3.0 4.0 6.0
Microsoft Internet Explorer 7.0.5730.11
Microsoft .NET Framework 2.0.50727.1433
Operating System 5.2.3790