Hi @Andy King
The Backup-SPSite cmd is used to back up a single site collection instead of a whole farm, so it only needs 300 MB.
If you need to start a full back up, you could use a cmd Backup-SPFarm. The Backup-SPFarm cmdlet creates a backup for the entire farm or individual components within the farm. It can also be used to back up just the configuration settings of a farm to be used as a farm template for other SharePoint farms.
For more information about back up farms in sharepoint server, you could refer to:
https://learn.microsoft.com/en-us/sharepoint/administration/back-up-a-farm#use-powershell-to-back-up-a-farm-in-sharepoint-server
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