Can’t add any useful information for you but wanted to mention that this topic was discussed recently and @fd0 gave some nice insights about it.
Depending on your data you might end up with some similiar numbers for restic. One of my repositories as an example: the raw data is about 390G but restic uses only 355G for 480 snapshots. It all depends on how much your data changes and how well it fits restic’s chunk size.