Live file is 5x bigger than Backup

Running a solution on FMSv18. When the file is open on the server, it takes up about 10GB of disk space, but the backups are only 2GB in size. Presumably FMS is 'saving as compressed' when it creates the backups (backups are automated by FMPHost.com) - but I wonder if there's anything I can do get some parity between the live file size and the backups, or if I even need to? Just seems like quite a significant difference...

Are the files Encrypted At Rest? Are the backups being compressed? It's not uncommon for commercial hosts to compress the backups when they complete.

The files are 'regular' .fmp12 files, not zips, but I'm not sure whether they run a Filemaker based 'Save and compressed copy' on the backup files prior to presenting them. But regardless - I'd still like to know what causes the file to inflate in size like that - I guess it must be an index of some sort...