How does SQL Server calculate the initial size of a (compressed) backup?
How does SQL Server calculate the initial size of a (compressed) backup?

From BOL :

For compressed backups, the size of the final backup file depends on how compressible the data is, and this is unknown before the backup operation finishes. Therefore, by default, when backing up a database using compression, the Database Engine uses a pre-allocation algorithm for the backup file. This algorithm pre-allocates a predefined percentage of the size of the database for the backup file. If more space is needed during the backup operation, the Database Engine grows the file. If the final size is less than the allocated space, at the end of the backup operation, the Database Engine shrinks the file to the actual final size of the backup.
To allow the backup file to grow only as needed to reach its final size, use trace flag 3042. Trace flag 3042 causes the backup operation to bypass the default backup compression pre-allocation algorithm.

You should follow - Backup and restore best practices in SharePoint 2013.
Also, to me your VM having 2 CPU seems to be underspec for running a 70GB database (Not sure if there is only one database or multiple databases).