When creating the iSCSI LUNs from within the Synology, there is a 'Space Allocation' drop down that has two values: Thick Provisioning (better performance) and ...
A Thin Provisioned LUN takes up actual used space and asks for more only when you write data to it. For example, if you create a Thin Provisioned 100GB LUN and ...
I have an iSCSI LUN that was thin-provisioned (space reclamation ticked) and mounted as the V: drive on a Windows server formatted ReFS and use it for backups.