What

You suddenly find that your backup files are larger than your database.

Why

There is no way that a single full database backup file can be larger than the database size.  A full backup contains the entire database's data files that are used (see this test), and the active portion of the transaction log during the backup.  It is thus possible that the backup file is significantly smaller than the database size, but definitely it cannot be larger.

The reason for the large backup file is because the file contains multiple backup sets.  A backup file will contain multiple backup sets if you repeatedly back up to the same file, without using the INIT or FORMAT options.  See Cumulative backups for more details