Content database size vs site collection size

Check all sites in the farm

Get-SPSiteAdministration | ft url,diskused -AutoSize

Reference links- https://sharepoint.stackexchange.com/questions/88936/content-database-size-vs-site-collection-size

https://docs.microsoft.com/en-us/powershell/module/sharepoint-server/Get-SPSiteAdministration?redirectedfrom=MSDN&view=sharepoint-ps