sql server 2012 backup question

i just did a copy-only backup of a database that is 18 gigs in size with a log file of about 700 meg in size
the bak file is 29 gigs.....

backup type was full

options where back up to the existing media set
append to the existing media set.

does anyone know why it is so big... the scheduled full backups are only about 18 gig
my copy is about 75 percent bigger than the actual database itself
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When you select "append to existing media set" the previous backup is retained. So, you are doubling the size of the backup by keeping the previous one. From MSDN:
Backing Up to an Existing Media Set

When you are backing up to an existing media set, you have the following two options:

    Append to the existing backup set.

    To make the best possible use of the available space, new backup sets typically are appended to existing media set. Appending to the backup preserves any prior backups. For more information, see Appending to Existing Backup Sets, later in this section.

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