The size of the DB from sp_helpdb is quite different from the actual size i..e the size being used which would be the main consideration for backup's.
- Please compare the free space and the space used for the DB mdf & ndf files
- I would also check the NTFS cluster size on the backup disk's for such a huge difference
- Are both backing-up to SAN, I am considering yes from the size: Please check the SAN metrics
- Do both of them have the same hardware i..e CPU,RAM, SAN, Physical Vs Virtual server etc ?