It is not advised to manually delete files from moodledata/filedir/ without knowing what they are and knowing what you are doing.
Sounds like autobackup issues/settings ...
Install this plugin:
https://moodle.org/plugins/report_allbackups
Review settings for autobackups ... in order to run autobackups and have the ability to quickly re-coup disk space, advise sending auto backups to a 'designated directory'. That could be 'autobackups' in your moodledata directory ... you have to manually create that directory in SG's file browser and set permissions so that the web service user can write to it.
Does SiteGround offer an attached data device ... just for storage? If so, investigate that and use the attached data device as storage for autobackups.
'SoS', Ken