First you need to prune the data store. Then wait¹ 24 hours. Then do "Garbage Collect" to actually delete chunks and reclaim that space as "free". Both kind of jobs should run daily or so - automatically. Check this! More info: https://pbs.proxmox.com/docs/maintenance.html#
What surprises me is that you show a Dedup Factor of 1.00 - I've never seen this before...
¹ There are "tricks" playing with the system date to "simulate" that waiting time. I do not recommend to play around with this...