SQL Server backups are suddenly smaller
My first guess was wrong so I deleted that answer, the OP simply made 3 backups in the same file, while the last backup was the only backup in the file.

To avoid this issue in future, add with init in your backup command, so even if you press F5 two times at the same minute (your backup file has datetime in its name so it seems that you cannot have more than 1 backup in one file, but your database size is very small so the backup is nearly instant), the first backup will be overwritten with the second and there will be only 1 backup in 1 file.