The default setting in the app is to back up all the messages and call logs available on the phone at the time of the backup (Full backups). While this is a good option for quickly creating a new backup, it can quickly take up a lot of storage space if there are lots of messages and scheduled backups are used to created regular backups. There are a few different ways to reduce the storage:

1. Use a different backup type – create incremental backups or use archive mode. This can be changed from Menu > Settings > Backup settings > Backup type. More info at  What are the backup types supported by the app?
2. Only backup selected conversations. This can be done from Menu > Settings > Backup settings > Selected conversations.
3. Only backup messages sent/received after a certain date. This can be done from Menu > Settings > Backup settings > Withing date range.
4. Set the app to automatically delete older backups. This setting is available for each of the cloud accounts supported and can be changed from the respective cloud settings screen at Menu > Settings > Backup location. For local backups, the app can be set to limit the storage used, this can be set from Menu > Settings > Backup location > Retention.