Hi
I saw this issue once. A customer reported that their actual data on the server was about 25 Gb, but backup size was 125 Gb !!!!
It was isolated to an issue with shadow copy components . UNder certain conditions, SCC would backup again and again.
Customer engaged MSFT for the issue, and the fixed it by installing VSS rollout patch 2
http://support.microsoft.com/kb/940349
This might not necessarily fix your issue, just sharing my thoughts. Have you opened a ticket with support?