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

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 about transaction logs, but my DB runs on my laptop and there is no DB activity during the time of the backup. (It only takes 5 minutes).

My backup type is Full, my recovery type is Full, and I am doing Copy-Only backups. Any ideas? Much appreciated!