Backup Files growing quickly on SQL Server 2016 Always On Cluster
What type of backup are we talking about?

Full? This only include the log records produced while the backup command is running (basically).

Differential? Same as above, as for the log records. As for data, it include the modified extents since last full backup. I.e., it will grow and grow until next full backup.

Log? It includes the log records since last log backup.

Also, what options did you specify for the backup command? If you post the backup command you use, we have some something to go on...