Mismatch on "sizes" with regard to database backups

I have a SQL database on a Server. When I prepare a backup job, I am able to select the MS SQL Server Instance and I can see that the database is reported as 80GB. Yet when I run a Full backup, I look at the backup set just created, and it only shows the job as being 1.5GB.

There is no failures in the job log.

Thank you.