Why is SQL Server backup so much larger than the DB files?

Question

I have a SQL Server DB that is about 13Gb with a 1.6Gb log file. When I back it up, however, it creates a 50Gb .bak file! Why is that?

I have read some commentary here

Answer 1

In my case, I found that the 500MB database also had a 560MB full text catalogue attached which gets included in the backup file. I thought I'd add this despite another answer having been accepted in case anyone else is still stumped :)