The Get Info option in the Backup Archive screen shows you more details about the backup that has been made.

Backup Archive - Get Info option

You can see the individual files it has been made up of as well as things like how much space it uses.

This data is available even when the backup medium is not connected to your computer as it is stored locally in a database.

Backup Archive - Archive Details

If you compressed the database it will also show the compressed size (see compressed size at the bottom)

Backup Archive Details - Compressed size